I have two Kids, The oldest Three the youngest 16 months.
I would guess it really depends on the Child and I would never begin to tell any parent how to look after their kids but I have told myself (and the wife) that neither will fly until they are old enough to understand what to do if it goes wrong. I.E What to do in an emergency.
I think it's important for any passenger to be able to understand the need to be sensible and what to do in an emergency. E.G forced landing, are they old and wise enough to get out on their own and stand well clear?
If they are too young it could Turn an accident into a tragedy.
Sorry to be so sober, but they are my thoughts.