There is nothing wrong with a girl playing sports, and even competitively. Just add whatever is necessary to make it modest, and problem solved. The fact that they are interested in some sort of activity that doesn't involve fornicating, partying, drug use, concerts and so many other countless common worldly activities for teenagers her age.
So long as she is not competing against "other" boys, and just with other girls it is perfectly licit. Remember that these are habits that will help her much better bare with the hardship's of being a future mother. The more fit you are, the easier it will be to carry a heavy baby for a long time in your arms without getting serious back problems etc... The more that you can actively play with the kids, without being pooped out so quickly. The time investment is worth it, even my mother can better handle the grandchildren when she keeps an exercise regimen. If she would not do this, it would be simply too much. Remember that after being a house wife, you will very likely help take care of the grandchildren and this can be even harder then when you were younger. So this is why healthy habits, will go a long way, never make an excuse. There is always time to set aside 30 minutes - 45 minutes for a good routine, 4 to 5 times a week. You don't need a gym membership either for any of these things, I don't go to the gym. Just stick to calisthenics, and body weight exercises. You can become super incredibly strong just with the floor and a pull up bar. We are talking about gymnastics strong here, its all about the routine you do. I know my nephews and nieces, really enjoy it when I exercise with them. They always try to copy me, it is amazing.
I see it all the time, mother's would love to pay more attention to the kids or be more active with them, but they simply can't because they have 0 stamina. After mopping the floor they are sweating and they think that it is a total body workout! No, I am sorry it is not a "workout", yes its good and helps burns calories, but a workout is when you actually push yourself. The best way to have kids that are not obese while they are under your roof, and when they get out of your roof is a by setting the example first, and secondly by not ever eating junk food yourself. They can't eat what you don't have in a refrigerator, and always have prepared meals they can snack on whenever they are not with you. These things will insure that the kid can enjoy life a bit more, for life is already too harsh at the moment and to deprive them unnecessarily will only make them more bitter, when they talk to other kids who have unlimited freedoms.
St. Don Bosco said, "Do anything but sin." So if the activity is not sinful itself, then don't prohibit it, fix whatever it is that causes scandal etc... If its the clothing, that can be easily fixed. Femininity is already intrinsic within the psyche of the woman, so long as she has a good spiritual life she will be feminine. Externals do not make you feminine, its the interior life that does. This is why you might have really "girly girls" that become lesbians and "manly" guys that are sodomites, the root of the problem is spiritual not a lack of barbies or toy cars. Just because you are not sowing 24/7 does not mean you are not feminine. We sometimes lose sight of these things, we LIVE in a totally different world and modern sensibilities are totally different then previous ages. The solution is not to TOTALLY filter everything out of their life, including what is good. Rather, to immunize them with truth and to help give them the tools to think for themselves according to the mind of the Church. That means that generally speaking, to advocate asceticism as was practiced by the Desert Father's is not generally good advice... There is a reason why you RARELY see modern man practice asceticism as it was done before, our fast is eating 1 large meal with 2 small meals and we call that hardcore :roll-laugh2: . I know a pious mother who makes all her children fast on bread and water, during all of lent... I am for reals here... Those kid have a more hardcore lent, then most monasteries in the world. One of their kids is going psycho on his mom, and showing signs of serious mental disturbance. They even had to take him to school somewhere else, but this is what I am talking about. St. Therese of Lisieux is a great modern example. She was incapable of doing traditional ascetical practices, she was too sensible and Our Lord through prayer showed her the "Little way." She became a great Saint, and died young. That doesn't mean the traditional asceticism is not good, but just be aware that some people can't be able to bear it. Its too hard on them, and to focus so much on the externals and not first take care of the internal virtues first, then later the secondary means of perfection. So there you have it, we are a total different creature get that drilled in your skulls many traditionalist fail to understand these principles and you end up with children that become sociopathic when they grow up. All the while, you had the best of intentions, but it did you very little good.
I know some friends (brothers) of mine who are in their 30's now, but the dad was super conservative Novus Ordo. Dad never drunk a sip of alcohol in his life, but he was way too strict. Well it turns out all of his kids really resented him, because he justified everything from a Catholic point of view. So that the children all of them, turned out to be horrible Catholics at the end of the day. You did anything, even the remotest thing wrong and he would criticize them for it. Even now at least 3 of them returned to traditional Catholicism, and the dad is stuck in the Novus ordo. Reverently giving communion to the sick, people in prison's etc... I met the man (came to visit) for 3 days and spoke to him. He knew the CCC inside out, but he was just totally blind. Truly the man was what I would call self-righteous, and it was just amazing to see both the son/Dad in the same car. You could tell my friend could not stand his own father, even after his conversion to traditional Catholicism. So please be aware, that being too harsh is not a great way to insure a healthy relationship with your children. You might get away with telling them, because I told you so and I am your father. However, they are going to all run away once they are 18 and have a calendar to countdown the day that they will leave their home because they simply can't stand you.
What I see is a common problem is parents are overcritical about minutiae, but when it comes to things they should actually intervene they never do. This is when you know that these parents are descendants of the Pharisee's, because they strain out a gnat, and swallow a camel. Instead have great charity kids are not going to get "it" perfectly the first or even 5th time you explain something. Its a lifelong process, they learn best through example and will never likely ever pay attention to what you say verbally (if it is not backed up with example). The verbal stuff they will start getting once they are older, but if your children hate you then the verbal stuff will only serve as a way to rebel against you, instead of a means to salvation. Once a child loses respect to his parents, it is extremely hard to ever gain it back. 99% of people out there, do not live according to the intellect and this is why there are so few that ever find the truth, by just reading books. The truth seeker is a rare creature, remember that and this is why we must employ every single means possible to make the life of your children as enjoyable as possible. Let them have remarkable enjoyable memories, you don't need to be rich to have these things. All you need is a rich spiritual life and great joy, your children will be infected with this without even you having to consciously tell them.
Parents beware of the signs that your children have lost all respect for you, if they just totally blow you off no matter what you say (get a clue). This usually means that the parents are all talk, no walk, and that children can be able to detect the utter hypocrisy. Kind of like your Uncle who is smoking and blowing the smoke in front of your face, and telling you "its bad to smoke don't ever do it."
I have never seen, and this includes all my siblings personally see my own father ever do a push up, take a run etc... Yet, it always amazes me when similar parents who are in the same boat decry how their children spend all their time inside, and never go play outside etc... The hypocrisy... Instead of complaining how bad things are, then do something to remedy the problem. If one of your kids, tends to over-eat (I was never overweight as a kid fyi, but I have seen first hand the abusive behavior against them) then feed him a healthier diet where it is pretty much impossible to over eat and gain weight. The sort of abuse especially if your kids are going to public school, that happens to kids that are overweight and the stuff they go through can be quite traumatizing. Kids can be the cruelest if not raised with proper morals, and even when they are raised properly they are pretty bad. They are vindictive, back biters, call names to other kids to especially hit them where it hurts etc...
Alright hope, that even one parent or a future to be parent. Take some of this advice to heart. I especially wrote this for those individuals that are too hardcore and take themselves too seriously. Lighten up when it comes to your neighbor, and be strict when it comes to yourself. This is the path of the Saints.