If you tell your children that there is a tooth fairy or an Easter bunny or a Santa Claus you are lying to them. It is a sin to lie, so you should not lie to your children.
Yes, I think the best way to solve all those issues is to simply do an alternative. If you don't know an alternative, just go through Catholic principle's and make a good simple family tradition. Make it according to your own regional custom's and adapt it in such a way to make it more Catholic. In the US we have Thanksgiving, but this is not celebrated all over the world, there are different particular secular celebrations. So any of these can be adapted in a Catholic way, so that it could deepen their faith and it can also make some unique memories for the family.
There are many ways in which one can make secular celebrations, into religious ones. Most people party and celebrate certain things, because many times its the only time of the year that you can get people together especially extended family. You can make these opportunities to spread the faith to those who are outside of the bosom and unity of the faith.
Take the secular practice of celebrating a birthday, even though I personally don't like it. Out of charity, I simply thank everyone that bothered to remember and/or brought a gift etc... Its the gesture that matters at the end of the day, but an excellent way to make it Catholic. Is instead of celebrating the birth of the day you were born with original sin, celebrate instead the day of your baptism as your real "birthday." Another way to have an excuse for a party, is to have a small celebration of the feast of your Saint whose name you received. For us Catholics, we have an excuse to party every day when you come to think of it. The Liturgy is filled with so many amazing events of the Life of Our Lord and Blessed Mother.
Instead of the tooth fairy, have him have a devotion to a particular Saint and give him some money for it. Make half go to the poor, and the other half he can keep. This will have a double effect, of making him see that all the gifts we receive one must also be mindful of those who do not have even the basic necessities of life. I know several families that instill this sort of thing since a very young age. Where all the money they get for anything, a chunk of it goes to a good cause. You can see that later on, many of those kids are very faithful to giving their tithing to the Church and have a generous spirit to the poor.
Now with easter and the easter bunny, I would just state the obvious. Nothing wrong with giving them a chocolate bunny, or even having them egg hunting. So long as they perfectly understand, that it has nothing to do with the feast, but its a way to just have fun. The best way to make sure their is no danger of error, is to egg hunt a good week or even two weeks, after the feast of the Risen Lord. So that there is no danger of them thinking that there is any sort of connection. For if you have them egg hunt, on Sunday morning and have them do that each year. It can definitely make them think that the capitalist exploitation of this religious feast is somehow good/holy etc...
St. Valentine's day, I would say simply ignore it with another alternative. This is one of the more dangerous secular celebrations, for most of the time it means a date with fornication by the end of the day. The way it is generally celebrated everywhere, is simply wrong. Kids shouldn't be sending love notes, even if innocent to teach other. It leads on a yearly basis, that somehow dating between minors and/or a relationship in some way is tenable. Rather on that feast you can be able to celebrate the feast of that great martyr Saint, by doing any of the works of mercy with the children. Show them, that even though at the moment we are not being persecuted by blood for the faith. That there are different sorts of martyrdom, white, blue, green etc... Different ways in which they can self-mortify and thereby imitate the great Martyrs and confessors of the faith. Show them that real love isn't trivial and demands great sacrifices, even to the point of death. That love can be manifested in several ways etc... This would be a particular day in which parents, can teach about the virtue of charity as opposed to the false secular counterpart "love." How it is so trivialized and is synonymous with lust/passion towards a person of the opposite or same sex.
I gave some examples of some ways in which you can be able to celebrate, but you can be able to adapt it according to your own regional custom's etc... The point is that we can in the world, but not of the world. So that everything can be truly restored in Christ, as the Apostle St. Paul proclaims. There is absolutely nothing that we can't baptize and make it into a proper Catholic celebration. The liturgical life of the Church is a living proof of this, and the near universal customs of each country whereby the faithful have developed ways in which to have special traditions to better celebrate a feast day is proof of exactly this principle. So take the practice of Los Reyes Magos in Latin America, and how they open their gifts on this day rather then on Christmas. There are many ways in which one can adapt, some decide to open them up on St. Nicholas feast day. I would prefer to wait until January 6, as it doesn't take away from the importance of the feast of the Nativity.
The feast of the Nativity of Our Blessed mother is also celebrated in many places with a cake and singing. We truly party too much as can be evident, but its a good thing. It really helps us endure the hardships of life much better and even North Korea has national feast days, because they use it for wicked ends. So feasting can be used in improper and proper ends, most of the time it is used for good ends, but it is important to keep in mind that many times its used to celebrate wicked deeds.