An odd one, if you can prove your under age child is under 18 then the letter is effectively I would have thought null and void, and perhaps you need to politely inform Moorcroft of this matter.
As a minor cannot enter into a legally binding contract, therefore a debt collector would have no reason to contact them. They cannot legally owe a debt.
A simple letter to Moorcroft with proof that your child is under 18 should put this to bed I would think to resolve matters.