It is obvious that the clerk merely transposed the numbers on the ticket and then was embarrassed when it was pointed out. In these days of blaming everything on someone/something else I am not surprised at the reaction, but you should definitely bring it up to the owner. Parent can deal with child in whatever manner he sees fit, but the customer is his livelihood and he needs to know what is happening.
Having run a business and caught clerks stealing from the till, it is never comfortable firing someone, but sometimes it is necessary, even if they are family.