Kerre Woodham has a good column on the issue. Apart from the usual stuff about how it is bad for children to be taught all men are dangerous, she also relates how some people have said that when they were kids travelling alone they preffered to be sated next to a man. The reason it seems is the guy will grunt once at you and leave you alone, while often the women will feel a need to be motherly and talk to you all flight.
It seems there is an obvious solution to this problem – let the parents decide. When booking or ticketing an unaccompanied child, just ask the parents if they have any preference for whom their child is seated next to, in terms of gender?