Many countries consider whether something is a job based on what it should be paid (accounting for typical pay, minimum wage, etc), rather than what someone is paid.
This is to prevent abuses such as shipping in workers from a poorer country, giving them food, board and $10 a week for them to stack shelves, and claiming "It's not a job", so the not-workers don't require a working visa.
Similar judgements are also often made in relation to tax law, working conditions, etc.
I'm not sure it's actually a bad thing to let people work for free if there's a reason they honestly want to. As long as they're not trapped in some manner.
Plus it's tricky when it comes to something like 'watching children part of the day'. It wouldn't be considered labor if it was a family member doing it, whereas stacking shelves certainly would be. If I'm letting someone live in my house, why should it matter if they're family when it comes to having them do chores?
This is to prevent abuses such as shipping in workers from a poorer country, giving them food, board and $10 a week for them to stack shelves, and claiming "It's not a job", so the not-workers don't require a working visa.
Similar judgements are also often made in relation to tax law, working conditions, etc.