I think you would be fine with kids that age. If you get an A boarding pass, you will have no trouble finding seats next to each other. You line up by boarding pass number number, and then first come first serve once walking into the aircraft. There are roughly 60 possible people in each class A, B and C. Special needs folks board first, including people needing mobility assistance, children under 12 flying alone.
The Boarding passes are assigned first by super frequent flyers/business class/higher ticket price flyers (which there are usually none to Orlando on our flights, or maybe one), that accounts for A1-15, and then in order in which you checked in. So the first number after business class is A16 (and is often the first person on because there are so few flying at the higher fee).
Once all As are boarded, then families (who were not already in A), then Bs and then Cs. So you can't do any worse than right after the As (roughly 45 people deep plus special needs folks), and it is unlikely you will have trouble sitting together.
The seats are 3 on a side (with the exception of the exit row, where one row has 2 across, but you couldn't use that anyway with kids -- and sometimes in the way back one row has only 2 across). There is a flight attendant in the exit row so you'll know where you can't sit with kids. We have always noticed everyone is pretty helpful and usually can move around to be sure families stick together -- not 100% but 90% observation true.
You can check in online, and if you do that right at 24 hour mark, you almost always get an A.