That changed a few years back. Effectively the cut off for BA is 14 years old. If under that age then they can't travel alone and they need someone over 16 years old to be in loco parentis. So in the OP's case the matter resolves within the next 12 months since the older child will be able to do this. Sun-AIr is still 12 years old. In reality they were never solo travellers anyway, and though the TCP is a good idea it isn't a requirement if the parents are somewhere on the aircraft. There is a separate safety requirement for children under 12 to have someone next or near them to ensure seat belts are worn at the right time.