Originally Posted by Gold77

That's exactly it. They require an adult seated next to the child to put the oxygen mask on the little angel, hence you can only book the middle pairs. The workaround is to book your child an adult fare. I am under the impression that F fares do not have a child discount anyway so there is no reason to select a child ticket but I may be mistaken. Same for Avios.
As a family of 3 aren't you better booking an AEF or EFK combo anyway? with the A or K for you, of course