Complete red herring
It matters not one jot whether this technology be used on children or not, the whole debate should be about whether the expense of these machines can be justified.
Over the last, just for the sake of argument, twenty years how many passengers have boarded a plane at Manchester airport and hijacked it or posed a threat with anything they have carried on board?
Out of that, I'm assuming, extraordinarily large number, how many would have been prevented by the use of this waste of technology for technologies sake?
Main threat to aircraft is poor ground security, airport staff (especially sub-sub-...-contract cleaners who have access to aircraft) and hold luggage.
Spend money where it will make a difference, not just give a contract to friends of whoever is in power at the time.
I see a further downside to this as the security staff will be paying far too much attention to the body scanners and less to the carry-on scanners.