I'm having mixed feelings, personally.
Lots of her friends the same age have already done it. She's bought a non-permanent kit from asda that says it washes out after a few weeks. She's chosen a shade that will work well with her hair (naturally a lovely mid-dark brown - she's chosed a reddish shade that isn't too brash).
Part of me is happy to let her do it. I'm sure I was using non permanent dyes at her age or a year or so older.
Part of me doesn't want to. The packs now say that it 'isn't intended for use on the under 16s'.