These are very difficult jobs and are also quite dangerous, it's not like being a store clerk. Working in a meat packing plant can bear a toll on your mental well being.
As a child, you can't be well aware of what you're getting into, and these industries can be predatory to their employees (big surprise as to why they are understaffed), by not respecting labor laws or pushing the limits of them.
Want your kids to work forced overtime attaching dead chickens to hooks in a fridge? I don't think it's as beneficial as working as a clerk. I wouldn't even be comfortable with kids working in kitchens, since these industries are predatory as well!