In reply to Arcturus:
I'd have thought most 11-year-olds would be big enough for an extra-small sit harness (perhaps the DMM Tomcat, as mentioned above), and overall that's preferable in terms of comfort and freedom of movement. But depending on her size, it may be worth still thinking about a full-body alternative. I've used them on kids of 11 before, even fairly big ones.
Our skinny 10-year-old still very much fits a full-body harness, and I don't feel confident that she has the hips for a sit harness yet. The Petzl Ouistiti is designed for kids up to 30kg, and we still have a couple of cm spare length to go in ours:
https://www.ukclimbing.com/gear/climbing/harnesses/petzl_ouistiti_kids_harn...
Post edited at 09:44