Why do my in-laws treat us this way?

I'm sorry to hear your story and i know you really just looking out for your kids probably. Remember though many children don't know their grand parents and get along in life just fine. There is probably a whole history that you have not been able to tell us but all these things make a difference and there are always two sides to a story. If it would really mean a lot to you for your kids to spend time with grand parents that don't want to see them, next time your boy asks to visit them get him to phone them and ask. If the story is like you say and you always try and they have no reason to avoid you..... unfortunately they need to want to change and seek advice, you can't make them. Sorry.