It's a grey area, I know children have guested adults but I have also seen it being declined if the adult did not also have access rights. My guess is the rule book says the latter but that custom and practice is to allow children to guest adults over 18 yrs in order for them to both enter the lounge.

Incidentally unaccompanied children can guest, and enter without supervision, for CCR and Arrivals, since there is no self service alcohol, so long as they are over 12 years old.