I dont think its a CAA reg. Children are required to sit with an adult (unless they are traveling as an UNMIN).
Tour operators use the words "Prebook you seats together" but together can mean with in one seat of each other. Worst case, you are in row 27F and the person you are traveling with is in row 26F. That is still classed as sitting together. Would you still pay £10 for that???
Again if you turn up early and are within the first 50 to checkin, you will most likely be able to choose your seat. In all the years of working for a handling agent I never saw a 100% presat flight.