Well, i've learned that one of the best ways to cope with things is to apply empathy to the subject.
If you can empathize with his decision to not tell his kids, that will help to relieve some of the hurt and that it feels personal.
I don't necessarily agree that not wanting to hurt their ex's is a valid reason. An ex is an ex, though you don't want to purposely hurt an ex, you have every right to live your life, and if you choose to be with another then the feelings of the person you choose to be with should be regarded.
Last time I let someone treat me a certain way for fear of hurting their ex, he went BACK to his ex! Ugh.
It's a tough situation, and you definitely have every right to your feelings about it. Once you have a discussion about it though, you may feel better that it's not bottled up inside. And then see how it plays out after. The only other choice I see, is to just do what said you want and try to stay out of it if you feel that would be better.