Widdly,

It depends on the airline!
We are flying JETSTAR this trip, as it leaves at 6:45pm & we can have had dinner by then & really no need to purchase drinks/meals after that time, the kids (fingers crossed) will sleep.
As Zyon is under 2 & Indah is not yet 2 on the way over they are Free but sit on our laps as such (it's only a 5hour & 50min flights so very bareable!)
Other Airlines you do pay something like 10% (from memory) & they also sit on your laps!

We pay for Indah coming back as she turns 2 whilst we are there!

I hope that helps...