When does 'having a joke at someone's expense (which happens a lot in life ) become bullying?
My answer to this is:
- If the "joker" does it to somebody knowing that they don't like it, perhaps if they've done it before. Doing it again is deliberately winding them up.
- If the "joker" keeps on mentioning it. For instance, if said shopkeeper said on the next visit "Now, you can wait while I just check the prices extremely carefully, I'll make sure I can add up this time...".
- If the "joker" pretends to be absolutely serious, and carries on doing so even after the other person starts looking uneasy;
- If the "joker" knows that there's a lot at stake. For instance, on results day: "did you fail your A-levels, are you going to cry?". In the OP's example, being in a shop and handing over the money yourself is a big thing for a child doing it for the first time.
- If the "joker" is about to go through airport security.
I think if the "joker" is in a position of authority over the person on the receiving end (to a small child, shopkeepers, teachers, police officers, doctors, parents, adult relatives are all authority figures) then that sort of "joking" is all the more risky, unless you know the person concerned, and how they might react. We wouldn't find it funny if cabin crew started telling us we didn't have enough fuel to get to where we were going so we would have to do an emergency landing, even if their next sentence was "happy April Fools Day".
To balance the argument a bit, here are some things I remember adults saying, all before I was seven years old, which I knew were meant to be funny. Although I usually saw the funny side straight away, I still remember that moment of hesitating to "check" it was a joke:
Teacher: "I haven't told you to tidy up yet."
Me (aged 6): "She (another pupil) told me to start tidying up."
Teacher: "If she told you to put your hand in the fire, would you do it?"
(If teacher had said "well, you'd put your hand in the fire if someone told you to", this would be far worse.)
Dinner lady on first aid duty: "With all your falling over, you keep breaking my playground."
Teacher: "...or you'll find yourself in deep water. Hot!"
Shopkeeper (handing over 1p change): "Don't spend it all at once."
Dad: "If you sit that close to the fire, you'll get your feet burnt off, like Pinocchio."