I am divorced with two kids age 11 and 13. My new partner is also divorced with two kids age 14 and 16. He separated 4 years ago and divorced 3 years ago. He still spends his ex wife’s birthday dinner with her and his kids. He says it is for the kids. I go through a wave of being cool about it and then think why? Fully understand that he would spend his kids birthdays together with her - but why would he want to have dinner on her birthday? Or am I being mean and not thinking about his kids. From my side I wouldn’t want to spend my birthday dinner with my ex (and we have a fine relationship - it’s just not something I would choose) and I find it rather odd she wouldn’t want to spend her birthday dinner with a friend. She has not got a new partner and from what he tells me she didn’t want to split in the first place.