Arnica can not be used as a precautionary medicine. If she has large bruise which is painful you can apply arnica tincture (different than arnica 30) externally on that part. Children are bound to get minor injuries while playing and you need not worry unless it disturbs her routine
Next Steps
never buy any medicine (allopathic or homeopathic) over the counter and ask pharmacists for advice or dosage. it is not their job to do so. which medicine to be given for which condition and in what dosage can be fixed only by a qualified doctor. never purchase medicines unless adviced by qualified doctor. this instruction is for children as well as adults
Health Tips
never give your child any medicine purchased without proper consultation be it homeopathic Ayurvedic or allopathic.