We've done breakfast at both and enjoyed both a lot. With Alice at 1900 Park Fare, the boys might get a little embarassed (especially because Alice likes to pinch cheeks while taking pictures with the kids). You do get a family picture before you enter into the restaurant so there is some "staging" that goes on. All in all though, it is a pretty decent buffet.
BUT...
Cape May Cafe is arguably one of the best buffets, not only for its food, but also the character interaction and the overall atmosphere. If you haven't been to the Beach Club and Boardwalk area, it's an added bonus to see this more "hidden" side of WDW. Considering the ages of your DS's, you can't miss with Cape May Cafe.
(Also consider whethere there is any impact on where you're going that day though - logistics is very important at WDW after all.)