One of the outputs from the BOAC B707 "Whisky Echo" crash at Heathrow in 1968 was that children should be seated with parents. Best safety is not achieved by seating them on their own and it is not fair to adjacent non family adults that they should be expected to reduce their own chances of successful evacuation and survival by following most peoples' natural instincts to help any child in an emergency. As result all families were then pre-seated together as a matter of course. This is a safety issue and not just a commercial one around maximising revenue per flight.
For similar reasons an Easyjet Captain ex Bristol a few years ago, faced with an infant whose car seat did not fit the aircraft seat and whose mother was already holding a younger infant, refused to allow an adjacent passenger who had nothing to do with the family to agree to take the second infant on their knee even thought a seat belt extension was available. Right decision though not popular with the passenger or press at the time.