Shellac lasts longer than polish if the child doesn't pick at it. It depends, just the same as with every client.
Shellac is. Not harmful and no work is done on the nail other than cuticle removal. The Shellac would be applied to non living Keratin and there is no medical reason why for a treat, it could not be worn as long as everything else is in place such as insurance and parents permission. I would say its no more risky than normal polish if applied by a professional and children have been playing with polish for years.
I am a professional .. which is why knowing my products as well as I DO, I know that no child's health is at risk from wearing a UV cured polish. I also fortunately do not live in a litigious society where every manufacturer goes way over the top to protect themselves from law suits. It's crazy .. even on the TV adds they list all the side effects and issue legal statements when advertising any kind of drug. Unbelievably boring.
It's become a way of life in the US to go to the law over the most minor of issues, so I'm thankful I have a good mind and can be professional in my recommendations.