Claire- FWIW, I think its totally reasonable for kids to speak to both parents on a holiday - I don't think it pushing yourself into his holiday - you're participating in your daughter's holiday in a reasonable way. My STBX facetimed the girls yesterday (without arranging ahead of time), and while I really don't want to hear his voice right now (I had to leave the room), I didn't even think twice - its important for the kids.
I would bring it up in mediation and get their perspective on how to handle for the future (schedule a call ahead of time etc.). I think some of these details can be emotionally charged and having a neutral third party can be helpful. My .02
