There are lots of useful travel-planning sites around, but none tailor specifically for kid-friendly holidays. MiniTime offers detailed reviews and recommendations on child-friendly places to stay and things to do, with ratings according to your kids' ages.
MiniTime is like a giant filter for TripAdvisor, where most of the content comes from. So instead of combing through reviews trying to find out if a place is good for families, you get recommendations from parents with kids the same ages as yours.
You can also book a hotel directly on the site. It's helpful for finding a trip that will be fun for the kids -- and keep you interested too.