Second to his kids!

I’ve been with my bf for about a year now. We are expecting a baby now. He has two kids and I do too. Every time we make plans something as simple as a kid free weekend his ex wants to ask him to watch the kids on her weekend! I always get my mom to watch my kids and never have plans to change that because I’ve already planned for a weekend with him but he will completely drop everything to get them! I’m seriously all for him getting his kids everything opportunity he can but he doesn’t understand that there are times he can tell her no I already have plans is okay. Am I wrong?