It's a shame about the cupcake. I'd have just encouraged them to eat one and join in, and expect you would have too, if you'd been there at the time.
My husband and I are vegan, but he's allowed to skip it if he wants to when we're out. He doesn't often, but he knows he can. And he knows why we are vegan at home and chooses to be that way.
I expect your kids will grow up and eat a lot of plant foods because it's how they've been raised. They'll be healthier for it, and might go vege or vegan if they want to, but maybe choose to be more flexible when they're out.
My husband makes vegan cakes by the way. :) Banana is a great egg substitute. And soya milk has been our staple for years. 😃