If you are talking about parents and children sitting in a commercial aircraft which is UK registered then there is distinct and clear guidance from the CAA that they should not be split up. I quote...

Seating – family
CAA guidelines ask airlines to develop procedures for the seat allocation of family groups, particularly when a group includes children. It is probable that family group members would seek each other out should an emergency evacuation be required, an action that could adversely affect the rapid evacuation of the aircraft. Children should be seated where they can be adequately supervised by an accompanying adult in the event of turbulence or a decompression in the cabin.

Children, accompanied by adults, should ideally be seated in the same seat row as the adult.

In wide-bodied aircraft, children and accompanying adults should not be separated by more than one aisle. Where this is not possible, children should be separated by no more than one seat row from accompanying adults.

FT reported facts:
A well know airline has a habit of not sticking with this guidance.

Happy landings
Spottie