We have to do d10's b-day skating Saturday that I set-up, so we have to get together at least. Maybe we can go shopping together for presents- (yeah, sure)
Maybe it's just me, but I look at it this way; if the mother & dad are going to live apart and have separate lives then they need to do their own celebrations for the kids. Why go shopping for the children's BD present and buying for the party, etc., as if it was all one happy family? To me, that is letting her eat cake.
Guess it's too late this time, but you need to think about that for the next event. From now on, whenever she wants the kids to have a party or whatever, then she needs to give it and provide for it on that side of the fence.....and then if you want to do something, you'll do it from your side. Sounds kind of cold and maybe doesn't seem like DBing, but I hope you see my POV.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!