We have been the last two years and going back next year. DS1 was 9 and DS2 nearly 7 the first year. The first year we took DS's out of school for the last 10 days of the summer term and this year we went May/June half term so they were out of school a week. Next year different story as DD1 now in year 6 and he has sats so we have to go in summer hols which is costing us £1000 extra on flights.
Personally, I would wait a couple of years as Florida is not a relaxing holiday as such - it is very tiring walking around the parks and believe me they are vast. You can rent single or double buggy/strollers at all the parks so your children will be able to get some rest walking round.
When we went in May/June this year we did not realise it was the love bug season and although they do not bite you they are constantly landing on you. Apparently this was one of the worst years for them. The weather at that time of year was a lot better than in July - not humid or as hot.
The children will be in absolute awe of the characters at Disney etc and it is really fabulous. A lot of the rides in May we did not queue long for but in July you could be queuing for 90 mins plus. (Friends of ours usually go Oct half term and this is another good time to go).
We stay at the Sheraton Vistana Villages on the really quiet end of International Drive. We have a 2 bed apartment with a full kitchen washer and drier. On the days you veg out you can just pop down to the supermarket (2 mins away) and get something really quick and easy to do. In fact when we go, we do not take any toiletries etc as we go straight to the supermarket and do a shop loading up on big bowls of fresh melon and tropical fruit and drinks for the kids (beer for DH, wine for me) cereals and treats.
Next year we are going to swim with the dolphins at Discovery Cove.
I think the key to success of going to Florida is do 1 day park, 1 day by the pool so you can recover for the next day. Always take snacks for the kids to the parks as they are expensive. Have a good scout round on the web for park tickets as we got buy one get one free from Universal this year. Get into the parks early. Hire a car. Fly Virgin - they are the best!
Hope this helps.