go for tampa. the amount you save on flights is worth it. can you take them out of school a day or two either side of the may half term as it will be a bit cheaper. worth checking whether private schools have the week before or the week after as they often have 2 weeks at May and flight costs reflect this. Easter is v late this year might find it cheaper then.
Scheduled airlines don't tend to come out until 11 months in advance. But if you do Thomas Cook/Thomsons then they tend to be around 15-18 months in advance. The offers came out last week and Virgin seemed to be the cheapest.
A lot of people have recommended Charter Travel too for getting holidays in advance booked earlier.
Flights tend to start appearing around 11 months before departure. The later you leave it the more expensive it can be. Also such seats can and do sell quickly.
You would also have to pay the ESTA fee for every passenger in your group. You apply for this online using only the official US government website and the fee is $14 US per application. This is valid then for 2 years.
The only carriers that fly directly into Orlando are BA and Virgin; do price compare both airlines before booking as there is usually some price difference. Another way of going to that part of FL is to fly into Sanford which is about 1 hour or so further east of Orlando.
Bear in mind that May half term is only a week long usually. Its a long way to go for potentially just a week.
Would love to take the kids to Florida for the May half term next year but not sure if we can afford it. The flights are not yet advertised for those dates, so can anyone clue me upas to a) how early can you book to get the cheapest deals and b) what would I expect to pay for each return ticket?