Would anyone know the difference

A father took his two boys to play mini-golf. At the ticket office he wanted to know the price.

- Five coins for adults, three for those over six years. Under six years entry is free.
- One of them is three, the other seven. I’ll pay for the oldest.
- You are silly – said the ticket seller. You could have saved three coins, saying that the oldest was under six; I would never have known the difference.
- That may be, but the boys would know. And they would remember the bad example for ever.