Actually, children under two are the easiest to deal with, as they sit on the parent's lap and do not need a seat of their own. EasyJet boards (or should board anyway) families with children aged 5 or less first. That age used to be higher but was changed because allegedly "large families took advantage of the system".
I hardly ever have a problem putting families together, even when 80% of pax are families If the kids are around ten or older, I don't think anyone has a problem with sitting a few rows apart. Actually, they seem to quiet enjoy sitting away from parents.
Hope this answers a few questions.
B