"By not seating a family together the airline is really expecting the PAX sitting beside the young child to take responsibility to ensure they are looked after in an emergency, helped with feeding, ensure they don't choke, are keep entertained and stopped from annoying other passengers."
LOL - I should try that next time I fly with my 3 year old daughter and 6 year old sun to Europe. Last time with my daughter alone it was 9 hours westbound back home (US) - and the on-board entertaining system didn't work...
But seriously, I have had seats reserved, checked in early only to receive new boarding passes upon dropping off the luggage - change of aircraft. In this case (I believe UA) they managed at the gate... So you can do everything correct and still...