C.S.
I noticed that one of my other granddaughters had the same problem when on a visit there. I've been through a couple of generations of long hair and have some simple suggestions. I know she is small, but I got some extra wide tooth combs to start with. It helps to begin at the ends of the hair about an inch up and comb down, then move up a little until you reach the scalp. You can "hold" the hair and protect the pulling effect if you start at the bottom as well. Conditioner helps too, but they make a spray-on detangeler.
I also use a plastic tooth brush with protected tips the same way as a wide tooth comb. If you are still having problems, work your fingers gently through her hair before you use the comb.
Sometimes new combs are sharp on the tooth tips. I use a fingernail file to smooth them when they are new. Hope this helps!
C. S.