Christmas presents to niece and nephew?

I have a question about giving my niece and nephew a Christmas present. I am going to have them spend the night this weekend and was planning on giving them a Christmas present. Here is the twist: My mother in law currently has custody of them, and she is a Jehovah's Witness, so she doesn't celebrate Christmas. I give them gifts all of the time, they are just not wrapped in Christmas or Birthday wrapping paper, and she is fine with that. As far as I know, she is not raising them as Jehovah's Witnesses right now, because soon they will be going back to their mother's. So my question would be- Would it be undermining her if I were to give them a Christmas gift?