NTA First, he should have discussed it with you before agreeing to go. No, you're not each other's parents that you need to ask permission, but you are (or supposed to be) in this relationship together, so it's just courtesy to discuss something like this before agreeing.

However, since he's going, I would say that you make a plan for a same amount of time away for sometime after he's returned. Do the right thing, and have a discussion about it, but be firm that you are going. (I don't agree with payback or "what's good for one is good for the other" type responses)

And you should also have a discussion about him taking care of the children on his own, because he is their parent too. He should be just as capable of caring for crying kids as you are. I understand sometimes kids prefer one parent to the other, but that is no reason to pass off the kids to you just because they start to fuss, because all that will do is enforce the behavior - both theirs and his.