I agree, get everything done, ie car loaded etc, then wake them and get them dressed, but not too excited, as there may be a chance they can fall asleep in the car again!!
Once you get to the airport, give them breakfast once you are through passport control, by which time you should have only another hour? to wait, so go and buy your last minute bits and pieces, or there is a very small soft play area at Gatwick they can bounce around on.
If you make them up a small ruck sack, with loads of pockets in it, and hide some new things in each pocket, ie, small colouring book & pencils, a new game boy game, a story book, some jelly sweets, a portable story book tape, magazine...........
Even if they still get bored they can swap and look at each others!!!
Dont forget you may get a meal onboard and possibly inflight tv.
Just dont forget, they may want/need an early night/afternoon nap on arrival!!!!