Hi there,

since you're heading to Varadero, I assume that you'll be going to hotel resort. There will be buffet style food and your kids should be more than ok.
I've been to Cuba a couple of years ago and we did some touring, although very interesting from a learning/cultural point of view, I would not advise anybody to do it with children as food may be an issue for them.

PM if you want more details

Cheers