I'm also horrified when people suggest drugging their kids to try to get them to sleep on a flight. So does it follow that if your child is being a bit hyper and naughty at home, you'll give them an unprescribed drug so you don't have to suffer their antics?
And I know that some doctors even prescribe drowsy-formula antihistamines for flights - I think that incredibly unprofessional, personally.
I'm about to do an economy class flight at Christmas with 4 boys aged from 6 mths to just turned 7 years. And I know my husband will sleep for about 10 hours of that 12 hour flight. And I know I'll sleep for about 30 minutes and will be comforting tired, whining children for much of it. Well, c'est la vie! I wouldn't dream of drugging them up so 12 hours of my life are a little bit easier.
Cara gave some great suggestions for activities. To be honest, we used to take a bag-load of stuff, but now find the iPad and the movie/game screen on the headrest in front of them occupy them for enough hours - and the rest of the time, they try to sleep or at least rest in the dark.
I would add a mini-jigsaw puzzle (about HK$20 from Wise Kids) as another activity. We also (used to) take play dough, crayons and paper, Aquadraw pen and book, mini plastic dinosaurs, a couple of toy cars, origami papers...
Best of luck - it's only a few hours and 'this too will pass'.