All the time. Much too regularly for it to be a mistake. Not just my dc from talking to other mums I know it's a real problem.
I know of at least 2 older kids who have short changed from £10 when sent there to get bread/ milk by mum. In both cases mums have had to go back to get their money as children were fobbed off by shopkeeper.
My dc is now wise to it and I tell them to check change and ask for a reciept. I also try to avoid them going there but it's not easy as there isn't anywhere else so close.
I am so cross,though, the shop is just over the road from school and I know they get a huge amount of passing trade on the way to/from school also it's just off our estate with no roads to cross etc so is really safe to let dc go to on their own to give them some independence.
I think it's just too cynical and mean to fiddle kids out of a few pennies of their pocket money.
I feel like wanting to tell the police but I guess they will think it's trivial and in a way I agree that it's not the crime of the century.