My DD has been using deodorant for over 18 months now but so far no other "signs" except greasy face.

I buy her a rexona branded roll-on marketed specifically for girls (you'll see it at the supermarket)...I seriously don't think my aluminium free number would do the job on her ponky pits. She's also got a can of spray on (impulse?? or similar) that she chose the fragrance for when she feels she needs a freshen up between showers.