Telkom Payment method added for google play store tooooo easy..

I have a R150 a month mobile account for a 10 year old child just to be in contact or for emergencies. Then this month I receive a R2000 bill....A 10 year old added a telkom payment method to a google account and bought games for almost R2000 it's ridiculous. Im expecting the other R1500 for next months bill. I called Telkom and asked how it could be so easy for a 10 year old to add a payment method on his google account. They just said the kids play around and get too clever.. How is there not more security.