It's up to you, but it's safe. She can lock her photos so no stranger can look at her photos. She can follow who ever she choses to follow and if someone wants to follow her she can accept it or decline it :)
I don't see why not. it's mostly used by children, teens, and photographers. you can also have restricted access to her account so that way people will have to ask to follow her. :) It's pretty much the photo version of Twitter, but much cooler. :)
Instagram is a photo sharing site designed for taking pictures of yourself, or of things that you think are cool. Its like twitter, and facebook pictures. were blended together and the concoction was instagram. Basically, and 9 year old should not be on instagram.