Is it fine that I still help my girls wash their hair?

My girls are 8 and 13 years old. They're multiracial and have extremely thick and curly long hair. especially the older of the two. They are both very independent , they just have trouble washing their hair and rinsing out all the shampoo on their own. Both of my girls have 2 completely different hair types, but both have hair that becomes too knotted and tangled. Cutting their hair doesn't seem to help when it comes to styling, it just seems to be thicker. My oldest doesn't mind me helping her wash her hair, she wants me to. On her own she seems to miss spots and leaves the shampoo in and I end up having to help her anyways, she just has so much hair!!! Also when it comes to styling on her own she gets frustrated and upset easily and she cries. She hates when her hair becomes frizzy and she says she gets made fun of, she even says " why's my hair look like this??!!?" Considering I have very straight, fine hair I think sometimes its hard for her to understand. I will help her style her hair or have her older sister help her . She knows how to style her own hair, it just takes longer than she wants it to and she just doesn't have the patience and gets upset and asks for help. Do you see anything wrong with me helping them?