Hyperhidrosis can be a devastating condition to deal with in social settings whether you are a child or an adult. Botox can be a viable option for both children and adults. I would spend more time with the parents and child in question to make sure they understand how Botox works, and the possibility of requiring treatments 2-3 times a year. Depending on the location (e.g. armpits), I would also offer alternative effective treatments such as miraDry. Depending the child's age/weight, I would also recommend starting with a lower dose with the option of adding to the dose if the desired effect is not achieved .
the treatment is for people suffering from the disease of primary hyperhidrosis sufficient enough to cause problems and may be used in adolescents but perhaps it would be best to start with a reduced dosage and work up from there...no inherent problem using the drug even though the patient is youthful...although Allergan notes the safety and effectiveness of botox under age 12-18 has not been established