The biggest problem is not your ILS, it's your H. He needs to be on board with you. Is this a gift for his child as well (in which case, he gets to have an opinion as well, as long as it is HIS opinion, not just mimicing his parents). If it is for your child (not dh's), you get to decide. If it's for his child (your stepchild), then he gets a bigger vote.
You can also always say "you can play with XXX toy when Daddy is here, but not when he is not around because I will not monitor you." Ditto if your ILS give you a gift you don't approve of "wow! that's a great present to play with at grandma and grandpas!"
I would put an end to the "just because your childhood was deprived....." argument. That doesn't open discussion and an exchange of ideas. That is a put down to you (you didn't have, and that is why you don't want your child to have).