we traveled for years with 2 small kids and never had any problems - if they've never done it before they can find it endlessly fascinating to twiddle all the knobs, actually look at the in-flight magazine and they even like the food...
Now many entertainment systems have e-games on them as well as cartoons - heaven!
TBH I'd ban people taking very small children on their laps for long flights - it's not good for the adult
But make sure you have a drink/food to hand when the plane is climbing/descending - they need to be forced to swallow to equalise pressure