This is a difficult one. On the one hand, if you pay for J or F you (rightly or wrongly) expect a bit of peace and quiet. On the other hand, if you want to travel J/F and take your kids along too, why should you be banned from J/F? I have taken my 3 year old in J, he was mesmerised by the gadgets, no problems. I think a compromise might be something along the lines of "must be over 5 to travel in F cabin or (say) 2 to travel in J cabin, unless you book the entire cabin." OR the airline could operate this policy only on certain flights, to avoid a total imposition as outlined above.

At the end of the day, it is public transport. No guarantees.