9 is WAAAY too young to be dressed like that. For their protection, they shouldn't be dressed or have clothing bought to dress like that. There are enough sexual predators out there to put the child at considerable risk (perhaps this is my paternal instinct kicking in, but one predator is one too many.)
Before the teen years, it is on the parents as purchasing parties not to buy this. To be consistent, it should proceed through the teen years to age of consent for the respective country (18 for the most part in the US.) As much as the need for teens to start discovering themselves and experiment, it is on the parents to discuss and enforce some decorum. After the age of consent or independence, then they can have full access to dress how they please.