Ditto the first poster. If you want to spend any amount of time at the beach, plan on packing up from Orlando, and staying part of your trip in another location.
I can tell you that getting up in Orlando, driving to a beach and spending the day, and driving back to Orlando to spend the night, is not going to be fun, fun, fun, with kids. And once you drive all that way to get to the beach, you (and the kids) are not going to want to only spend an hour and then get back into the car.
Are you flying to Orlando, or driving? If you are driving, then I'd suggest you just leave a day or two early and stop over for a day or two at a beach on the west side of Florida on your return trip home. Driving more than an hour to get back "home" to a bath/shower after leaving the sandy, sweaty beach is no fun.