Like everything else the firsts are the hardest.
It will become your new normal and for your kids too.
The sad clown and I sat together at my big girls first day of school - mainly because my then 2 year girl old saw me and wanted to sit with me so he slithered over next to me.
There haven't been many other shared events but we seem to have managed to stay out of each other's way with the girls hanging out with one of us at a time - or one girl each at a time which is a great given how rare 1:1 time is in a single parent household.
It has become easier. I've let the other mums in on what happened and they're also aware about the shit he pulls like not forwarding paper invitations to me for parties that fall on my weekends. They now all know all about his fuckery so they call me to make sure I've seen the invites and/or send me the details.
Soon she will be as invisible to you as that unpleasant colleague from out of state that you only have to see at big work events.
I wouldn't insist on the kids sitting with her every time. I understand it for the first time but I wouldn't make it so every time. I'll have the girls with me for a bit then I'll encourage them to go sit with their dad.