First, is it actually 3 hours? If you land at 12pm for example and the next plane takes off at 3pm you won't be off the plane until after 12 and you'll be boarding by 2:30 at the latest. If there is a delay coming in, even less. So keep that in mind.
But there are plenty of restaurants and shops in the area and between books, games, bathroom break and grabbing something to eat, it goes by fast. My kids at that age love to just watch the planes too. We do a round of I Spy or charades and let them draw and it's rarely an issue. Take a walk when need be. Security can be such an issue at MCO I don't think I'd want to have to deal with that to get back in. We've easily spent 45 minutes to an hour in line for security some days so that would eat up most of your free time.