I am not married, so I'm a bit of interloper here.
Hi and welcome.
My first thought was the same as Serenity's. What is the underlying issue truly? Is it about "taking care of his kids"? Is it about not having to raise anymore kids (since you've done that already with your own)? Is it about having young kids in the equation again? Sit down with yourself and really think about the situation. What is the trigger? What is making you feel resentful?
i.e. what about having to buy buns set you into the resentment mode? Was it the actual buying of the buns? Was it that he's treating you more as a GF and you don't have that official title yet and want it? Is it that you feel he's expecting you to be a "mom" to his kids? What was the real reason for the bun trigger?
Only you can truly answer that. Honestly, at this point, I think you need to figure out your end of things before talking to your BF.
BTW, he sounds like he's got everything balanced. Which is a good thing.
My 1st thought was that so many of the women on this board would trade places w/ you when you have nice kids who like you and no problems w/ the ex.
Your BF seems like a good guy.
Since there are a lot of days when he isn't w/ his kids, what is his excuse for not doing things w/ you then?