I'm surprised no one has mentioned this amidst the posturing: Certain flights frequently do not allow seat selection in advance and parties connecting with small children (or adults requiring care) may be inadvertently split, requiring someone to move.

IRROPs happen too: glitches in the reservations system, delayed/canceled flights, etc. There are many reasons people traveling together could be split up through no fault of their own and it's not reasonable to separate small children from their parents or the disabled from their caregivers.