2cruise, please don't sweat dress-up night. There will be people dressed all different ways, and, in general, kids and teens aren't as dressed up as the adults. The main question is, how will YOU feel if everyone at the table is dressed up except your kids? If they wear a collared shirt and tie, with no jacket, it'll be perfectly fine, and I promise you there will be kids (and probably adults, too) who are dressed more casually. Do plan to have the photographer take a picture of all of you. I think half the reason there is a dress-up night is so the photographer can sell more pictures of you back to you! If your kids' dress is ok with you, it'll be ok with everyone else.
Also, a LOT of people go back to their room after dinner and change back into casual clothes. I'm sure your kids will be happier with whatever they wear to dinner, if you tell them they just have to wear it through dinner, and then they can change.