Laura -
I'm sure lots will join in to offer their suggestions, but if you provide a few more details, you'll get answers more tailored answers.
What type of free ticket were you given - is it a 1 day/1 park ticket, or a 1 day park to park access ticket? To answer your question - there are 2 parks that are within walking distance of each other: Universal Studios and Islands of Adventure.
What types of rides are your kids into? Do they like the coasters at Disney? If so, which do they enjoy?
Are you driving from Port Canaveral to Universal on the same day that you'll be going to the parks? Are you staying overnight at either onsite hotel?