My sister has 3 kids now. She had at least 2 of them baptized in the Methodist sect and they are all under the age of 7 but one of them is turning 7 this year. I would like to know of good ways to teach them the faith so that they don't end up as heretics. I don't have my driver's license yet but when I get it I would want to see if I could baby sit my niece and nephews so I could teach them the faith. Does anyone here have any advice about how to go about this? I want to know since it would be a good opportunity especially considering how they are impressionable and if I could act before they fall into formal heresy.

Start by listening.  While interacting ask one or two questions "What did you learn at Sunday school?"  "Did you go to church last Sunday?"  
Find out what they think they know.  It will give you a direction to go.
Another approach....during a quiet time pull out your rosary and let them start asking questions
Leading by example is always good.

I have good results when I tell them stories, about the angels especially their own guardian angel, how it is always there.  I remember when I was a child and I would sit down on the chair for dinner I would make sure I left room on the chair for my angel.  Children also like to hear what heaven is like but to get them interested get down to their level.  Tell them about the mansions there and they will have their own.  How the streets are paved with gold and add a little childlike imagination such as the flowers might be lollypops. 

Next time they see you I guarantee they will ask for more stories, at that time you can begin with Adam and Eve and the slimy snake.  Just keep adding to your stories little by little, of course, teach them to pray each night to their angel, at least one Hail Mary.  When I was a child attending Catholic school, the nuns always instructed us to pray every night that when we were ready to die, that we would be united to His Church, and I did that not really knowing why, since I believed I was already united.  This was before VII started up!  Now I thank God, I obeyed the request.

Teach them also to ask to be united to the true Church the moment they die.  

My sister has 3 kids now. She had at least 2 of them baptized in the Methodist sect and they are all under the age of 7 but one of them is turning 7 this year. I would like to know of good ways to teach them the faith so that they don't end up as heretics. I don't have my driver's license yet but when I get it I would want to see if I could baby sit my niece and nephews so I could teach them the faith. Does anyone here have any advice about how to go about this? I want to know since it would be a good opportunity especially considering how they are impressionable and if I could act before they fall into formal heresy.

Live the faith, dress and act the part, and it will have a greater effect than anything you say. Do not play games with God and your own mind, never lie to the children. Stay in the sacraments. Do all of that and the children will someday be like you. They are not your children, so your "influence" time will be seconds compared to the influence of the world (their school teachers and parents).