Hi Cally - flights can be difficult. I usually keep an eye on them and wind up booking about 3 months out. Because you're planning well ahead, it would be a good idea to start watching 2009 flights to get an idea on how far out they'll come down for your time of year.
Do you have to go in August? I have been at that time of year with two little ones and the heat and humidity is intense. Plus the parks are crowded. I am going in June this year and that will be hot and busy enough.
If you could swing it, late October is a lovely time of year to go for nice weather and fewer crowds. But if you can't - any time in Disney is great
Back to the flights - Virgin are brilliant. They have a lot to keep kids amused with seat back TV's, lots of films and games etc. Some other airlines do have seat back TV's. When you're researching flights, check out the type of plane and look it up on seatguru.com to see if that plane has the seat back TV's. It makes the flight a lot easier with kids.
I try to fly with Virgin from Manchester when I can. But if prices are cheaper, I fly with another airline and sometimes get a regional flight from Newcastle if possible.
Changing planes in a US airport isn't too awful, so long as you allow yourself time between flights. And one bonus I have found is that you go through customs at the airport you arrive into. So once you get to Orlando, it makes things a lot quicker.
To monitor flight prices, I use sites like Orbitz, Expedia, Virgin's own site and farecompare.com