My DD1 is just 8 and has been brushing and tying it up herself for at least a year. And she washing it herself too. I would say nearly the same length hair as your DD. She just learnt how to do it herself so i can't actually give any useful advice.
Maybe get her a Rapunzel doll / wig so that she can practice on it?
My 6yo DD asked me to cut her long hair as she was doing a lot of swimming and it had all got too much: way too long. It's now mid back / below shoulder length. She puts bobbles in it and brushes it, we are working on her rinsing it out and shampooing at the moment.. all going well enough. My Dd practices on her dolls and we got her a long wig as well!
I'd get it cut much shorter so she can manage it herself and then (if she wants) when she is competent she can grow it longer again. There always seem to be far more children with long hair than there are adults, so I suspect that looking after long hair is far more hassle than it's worth.
Having said that my dd had a bob and still it looks like a birds nest half the time.