Some use this to prove that this was not a miracle. But the response to that is how did Jesus know that that particular fish had a coin in its mouth, that Peter would catch the right one and how did he know its denomination or amount. It seems that Jesus had not been at the Passover - perhaps up in Gentile territory. Therefore he wasn't around to pay the tax. Royalty goes free.
Sons are exempt.
They don't pay taxes. This is a religious issue - temple tax. All the citizens of Israel. The Son of God. After all the temple is God's House.