I think the point is that if only one parent is travelling, there's only one parent to help with food, entertainment etc. (stopping the child annoying other pax!), but if both parents are travelling it makes sense for everyone if both parents can help - maybe even take shifts on a LH flight.
The policy that at least one parent will be sat next to the child is OK if a late booking etc. (for operational reasons, BA may need to reallocate your seat assignment (to cater for parent/child late bookings), but when the cabin is wide open, both should be sat next to the child. Dad in D, child in E, Mum in F.
Simples!