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Calling now won't help - if there are no seats, there are no seats. The agent won't open up seats for families on most airlines.
You will need to check in at the 24 hour mark, arrive at the airport early, arrive at the gate early, ask nicely, prepare not to sit together, plan for one parent near one child, offer up drinks or other benefits to others, ask, don't demand, and
definitely do not
loose my mind if they didn't each have a parent with them
At best that will alienate the crew and other passengers, at worst you would not be permitted to fly (which did happen to at least two parents here in 2009)
On flights to MCO there are a large number of families with children and you may not find anyone else willing to move to meet your needs. That's the reality on full flights these days, especially flights full of families.
Edited to add: crashbb has faster fingers.
re calling to get them to open seats - as an example, on the Airbus twins UA holds 6D/E for special needs passengers, NOT for families. LH does the same with various seats on various aircraft. LH will sometimes give me the special needs seat
just before boarding if I ask nicely. UA opens up 6D/E to elites at the 24 hour mark.
UA may sometimes hold the very back row (non-recline, next to the lavs) for families; if you do call do expect to end up in something similar, IF the airline has such a policy and IF they still have empty space in those seats.