Just for some additional info, BA has some details here, at the bottom of the page discussing children in different cabins

Tickets and travel documents | Information | British Airways

Basically, make your own booking, then phone up and add a booking for the kids. (I note it says between 5 and 12 years old, but I can't imagine any issue with a 13yo as long as they are linked to your booking)

Unsurprisingly it says that if your child needs assistance, you will need to voluntarily downgrade during the flight, they can't come up to the higher cabin. It doesn't seem to consider the possibility of the children being the ones booked into the higher cabin in the first instance.....has such an arrangement ever been recorded