When ingame Currency is purchased for a game, it is locked to the purchasing Account, he won't be able to use it on his own Account as it was purchased on your Account... You could try calling Customer Support and have a word, see if they are willing to Refund... They don't normally, but it's worth a shot, they treat calls - case to case.
A Sub Account does not have it's own Wallet, it draws funds for purchasing from the Master Account (yours)... You can set a spending limit from the Parental Controls. He should be able to purchase items on his own Account up to his age limit.
Making a false Account like the person above is not really the best thing to do.. If that person has used a random date of birth... there won't be photographic evidence of proof for that date.. If Sony were to ask for it... The Account will be banned... all purchases to that Account, Trophies, game progressions would be lost.
And of course, it opens up your child to adult games and the misuse of Credit Cards. Kids don't think, they do.
Ava