TBH, Eli, it's really not that bad. S'pore is 12 hours out, 13 hours back (headwinds or something ) and we've done it with a 2.3yo & a 6mo, then a 3.3yo, 18mo & 9 wko, now it'll be 4.3, 2.6 and 14mo. Ds1 has got easier each time, he's content to watch the inflight entertainment for big chunks but we found taking lots of new bits and pieces (playdoh, fuzzy felt, colouring, animals, cars, etc) kept them amused.
The sleeping situation is hard because they can't lie down but they usually manage about 6 hours or so. Also, the cabin crew have always been great at coming by and chatting, supplying food (chocolate!) at stressful times.
Really, once you've got the take off out the way, then a trip to the loo (most exciting), a walk round the cabin, then a meal, you've passed a couple of hours already. You'll be fine!
Sympathies on the stroppy 4yo, too. DS1 is a little bugger challenging at the mo. HUGE screaming shouting tantrums when I make the tiniest request of him (usually 'turn the telly off and come and eat your breakfast'), up to 6 a day - grrrrrrrrr. I imagine Tanya Byron standing in the corner of the room saying 'ignore, ignore, ignore' .