Can small children be seated away from their parents/guardians on a flight?
This is what I could find. I looked on the Civil Aviation Authority's website and found this article.

Seating Allocation


  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.


From what I read, it doesn't look like there is a law set in stone, but instead it is recommended by the CAA to be put in regular practice by airlines.

If there was a law then Ryanair would have been in serious trouble for charging people extra  money to sit next to their kids.

Ryannair booking separates mum and tot