Hey, if you work retail behind the cash register, do what I do.
When parents are buying toys, or sweet treats for the kids -this only works if the kids are standing close to the register, waiting for their stuff - scan all items as normal, then lean toward the kid, hand out and tell the kid the amount £10, £20 etc. you're looking for to pay for it. "That'll be £10.20, please."
Parents are always amused by it. Even other people in line will smile. Kids, especally those 5-12 will ALWAYS get a surprised or shocked look on their face.
It amuses everyone.
Sometimes I'll add this; after the routine, when the parent pulls out the money, or the card, I will point to the kid and say "I see, your turn to pay next time?"
It works especially well if the purchase is 50/50 toys (or candy) and regular items grown ups buy.
Additional: you can mix this in: if the kid is asking for one of the impulse items they usually keep at the check-out (like chocolate,) or something else -for example, at my work place, we keep our really expensive lego sets behind the counter to prevent theft, and obviously the parent doesn't want to buy it, but the kid really wants it, just look at the kid and say "I can give you that, if you have (ITEM PRICE) and hold your hand out. Parents like that, because it always silences the kid.
I've been doing that to at least one kid a day for three months now. No complaints, and it always seems to amuse everyone.
Just a little something to brighten everyone's day.
