Ive sat near babies and toddlers who have been quiet and not an issue, but far more often on long hauls its been very distracting not just the crying (and sometimes full toddler meltdown), but having toys and food thrown into aisle then being constatly asked to pick up said toy/bottle and pass back, and constant opening of overhead bins to get kid stuff out or put it away, the pacing up and down the aisle sharing the noise with those trying to sleep, read or watch their IFE.
An idicator on the seat map and a few rows of prefeered adult only seating or mini cabin gives people the option to maximise distance from potential screaming kids - it is why i've burnt points over the years for exot row on A380 to gurantee i wont be seated next to a small child. Also why i predominantly plan personal travel outside of school holidays.
The worst flight of my life was a 6:30am flight to Melbourne on Ansett where there were 4 relatively newborns surrounding me on a full flight (why if travelling with kids not fly later in the day, they dont need to beat a 9am business meeting) - i was on aisle, had a baby next to me on window, one across the aisle, one in front and one behind, and all 4 screamed from moment of take off until landing, i thought my ear drums would burst. Never seen the call button pressed so much with male passengers demading to be moved, and FAs hiding cause nothing could be done.
As a parent of a young child, wouldnt you prefer the people sitting near you were sympathetic? How does people being easily able to avoid sitting near a kid hurt you?
Yes drunk and dykwia wankers present their own challenges, but to date ive never had entirety of a flight made unbreable by an adult. And yes i realise i have probably just jinxed my next flight.