[Kaytee]

well being a Christian myself, your parents are not acting Christian.
I see nothing wrong with them teaching your children about the Christian religion in a non threatening, non aggressive manner. Just as I would be fine with someone else teaching my child about non-Christian religions.
As I said though your parents are not acting Christian. They need to respect your religion, regardless if they believe in the same thing or not. THey need to respect your children as they venture on their road of religious discovery.
You are their religious guide, you are there to teach them your beleifs, if they choose as adults to go a different way, then that is their choice. But your parents need to be supportive of them.