Cut it short. Super short. I tell my kids, you don't want to take care of your hair? Then cut it short.
Make sure she uses conditioner after shampoo. Get some detangling spray or leave in conditioner spray for her hair after shower. I comb my hair when it's damp but not wet, and always ALWAYS start brushing from the ends of the hair working your way up to the roots. If you start at the top of her head you are just making the knots worse. You can also braid her hair after she showers and she can sleep with it braided making it less likely to tangle while she sleeps.
When I was younger, I always wanted long hair, but my mom wouldn't let me have it. I was too young to care for it, and she didn't want to mess with it, so I always had to get it cut short. I think when I turned eleven or so I was finally allowed to grow it out.
Buy a spray on detangler. As a parent to a child with naturally curly hair I can vouch it's worth it.
Get a wet brush or tangle teezer those things are amazing . I hated brushing my hair until I found them . They are the only things I can get through my hair .
Oil. Coconut. Argan. Any hair oil will help. Put her hair in plats to keep it tangle free.
My hair is knotty like this, parents shaved it still traumatized.