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You are rightly dismayed by this situation, but you've phrased the question curiously, emphasizing your discomfort rather than a child's well-being. If you are concerned about the daughter, as you admirably seem to be, you ought not make her life harder, which firing her mother would certainly do. And rather than insist that the mother make the daughter drop that wheelbarrow, you might encourage your sons to invite the daughter to play soccer with them. Her mother will likely be relieved: Having a 9-year-old "help out" all but guarantees the task will take longer.

It would be commendable if you could proffer some practical advice. Presumably this woman brings along her children because she has no alternative. Does your town offer inexpensive day-care programs? Are there other social services that might benefit her three kids? A bit of time on the phone or online might lead you to something that helps this family and eases your own mind.

UPDATE: The gardener failed to show up a few times, and Jane "used that as an excuse" to find someone else for the job.