That would be pretty good if it was a husband and wife rather than a father and son. The image of a man grabbing a boy by the genitals is a bit Savile-ish. and anyway HMRC doesn't squeeze kids too much. My edit:
One day at the local cafe, a woman suddenly shouts out: "My husband's choking! He's swallowed a pound coin! Help! Please, anyone! Help!"
A man at a nearby table stands up and announces that he is quite experienced at this sort of thing. He walks over, wraps his hands around the husband's genitals, and squeezes. Out pops the coin.
The man then goes back to his table as though nothing has happened. "Thank you! Thank you!" the woman cries. "Are you a paramedic?"
"No" says the man, "I work for the Inland Revenue".