That's a fairly long slog by bus between Cancun and Belize City, which will take you the better part of a day. (If you have your hearts set on visiting Belize, I'd recommend flying.) You might want to stick closer to "shore" and explore Cancun, the Riviera Maya, and Cozumel just offshore. Big Incan sites to check out are Tulum and Chichen Izta, the latter a bit further inland and a full-day trip. Have you seen Away.com's Cancun Family Travel Survival Guide? Might be worth browsing to get the lay of the land: http://away.com/features/family-travel-survival-guide-cancun-1.html