She may be able to avoid some of the self employment tax. The work she is doing might be classified as "household employee" income, rather than self employment income; the baby sitting income in particular. As long as she did the sitting in the customers home (not her/your home), it is household employee income. Since she is under 18, there is no $ limit on how much she can make.
If she used her/your lawn mower, instead of the customer's, it is less likely that she can call the lawn work household employee wages.
If she made less than $6350 for the year, she would pay no tax as a household employee. She would still want to report the income to be able to make a Roth contribution
