I normally fly Air Tran. My recommendation would be to purchase your seats (at least for the flight coming home). There will be alot of families on flights from MCO and most of them pay to sit next to their families and will not be willing to switch seats with you. Heck, for that matter I fly solo alot, but due to claustrophobic issues will not sit in a window or middle seat and will not sit anywhere near the back of the plane. Consequently I pay extra for those $15.00 or $13.00 aisle seats close to the front of the plane. Sorry, but even as a solo flyer I'm not giving that seat up, unless you have an aisle seat close to the front to trade.
Again, I fly Air Tran pretty frequently and it seems I always see people arguing with the gate agent that no they didn't purchase their seats but Air Tran must seat them next to their small child. The response is always the same.: "We did not separate you from your child. You chose not to purchase seats together and we will not force others to move to accommodate you. You may ask when you get on the plane if someone is willing to switch seats with you, but we can not make them." Personally, I'm not giving up the $15.00 aisle seat in the front of the plane that I paid for, for a middle seat in the back of the plane for a family who was too cheap to pay. Sorry, if that sounds harsh, but it seems I see it on every flight I've been on the last couple of years. If you choose not to pay for your seats, you'd better be prepared to sit apart. Coming home from Orlando especially, I can't imagine families not purchasing their seats, because almost everyone else on that plane will have. Even if the resort will check you in at the 24 hour mark, there is a very good chance that on the flight coming home there won't be seats together. Going down there yes, you may get a couple of seats in the back of the plane together, although my last few trips to MCO have been overbooked and even at the 24 hour mark, the only seats that have been available are middle ones scattered through out the plane.