This is what the CAA say...
Families, children and infants
The seating of children close by their parents or guardians should be the aim of airline seat allocation procedures for family groups and large parties of children.
Young children and infants who are accompanied by adults, should ideally be seated in the same seat row as the adult. 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. This is because the speed of an emergency evacuation may be affected by adults trying to reach their children.
Whenever a number of infants and children are travelling together the airline should make every effort to ensure that they can be readily supervised by the responsible accompanying adults.