I would buy one for her and tell her you found them on clearance. This way she would not feel like she has rules to follow, instead it was a gift. Plus you can use it for your future kids.
There are very few states in the U.S. with legal minimum ages for children home alone, but many state agencies have published guidelines. Georgia, Illiniois, Maryland and Oregon are a few of the states with specific ages mentioned in their laws.
12 years of age appears to be the most common recommendation. Below in Related Links is an article entitled Home Alone Children Legal Age Limits which provides a state by state comparison with references.
One guideline from a California child welfare representative suggests that 8 year olds and over can be left at home for a brief period of time (usually just after school).
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