I am sure one of them will ask me a question when they get older. Especially when they become more aware about relationships, MR, family, etc. When certain things come up in conversation I always stay positive to them about their mom. This morning I made sure they knew that the presents under the tree where from both Santa and mommy/daddy even though they were not labeled from us other than those from Santa. It is important to me that they feel no anger, ill-will, etc. because I went through that as a kid and I hated it.
I got her a present from the girls and she was definately surprised by it and I don't think she knew how to act. The girls were excited and I wanted to make sure the experience was positive. It felt good to take the high road and to show my W I am not an angry person. I also think if I was angry, showed resentment, etc. it would further re-enforce to her she made the right choice.