Wow. I don't know a lot of ex husbands that would be that dedicated and helpful when their kids are so sick. To me, this is a sign of a wonderful, caring person and father.
Yes, it sucks that he's with his ex wife but vomitting kids isn't a carnival.
I can't tell you how many times my relatives were shocked at the things I had to go through alone as a single mother. Helping with the water heater? Nope. I had to figure it out myself. Car broke down and no help? Nope. Broke my leg and couldn't drive or get anywhere. My ex wouldn't help me? Nope. A little help with groceries while I was on disability? Nope. Vomitting kids? HE didn't want to get sick.
I think your boyfriend is doing the right thing. It can be hard not to be jealous of time he would spend with ANY woman, but it sounds like he's checking in with you. You need to realize also that him doing the right thing doesn't mean he cares any less for you. He's got two special needs kids. What would you think of him if he was your brother in law and left everything to your sister to deal with? Hypothetically speaking, of course.
The most beautiful thing about a woman is her confidence in herself. It's not easy, but don't let sick kids get you to wondering about your own self worth.
Be positive. Tell your BF that you are glad he could help his kids and you're happy to have him back home.
He has special needs children so this is something that you will always deal with in your relationship with him.
He needs to know that you love him enough to trust him.
If you DON'T trust him, you know how that will end up.
Perhaps you love his children and feel as though you are left out of being able to care for them as much as you wish you could. That would be a valid feeling you could share with him.
I wish you the best. It sounds like you've got a pretty great guy.