Is it ever okay to leave children alone for hours?

My mom owns a house that needs a lot of work done. A guy from across the street comes to work on things. Like today he was replacing the doors. He was there for 2-3 hours. At one point he said that he is watching his daughter's kids because she was at work. So he left them alone all that time. Is that unethical. I don't know how old they are.

Is it ever okay to leave children alone for hours?
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