I'd argue that kids are not fit for the stress put on people in service positions with customer contact. It's fine if they have a holiday job cutting grass or delivering newspapers or something like that but standing behind a counter taking orders from people that often don't even acknowledge that you're human, too? That's hard enough on adults already - I definitely don't think it's the kind of job for kids.
Also which business is hiring kids to work a couple of weeks during school holidays and then is fine having one less worker again? The time spent on teaching the child what to do and how to handle different situations as well as the paperwork probably takes more time and money than not having the help for a couple of weeks - even less so as you probably have to have another person nearby in case of customers overstepping so I'm not sure this is just some holiday job for the kid to earn pocket money or get job experience