8yo dd has fine, straight hair that reaches her thighs. She can sit on when it is loose. Most of the time it's in plaits or an up-do. To make school-day mornings easier she sleeps in plaits, which means that I rarely have to detangle. I use tangle-teaser brush, or a very large paddle brush with plastic (?) spikes rather than bristles. If it's wet I use a wide-toothed detangling comb.
I buy hair serum, the sort that advertises shine and smoothness, and that you're meant to apply to wet hair. I dilute it in a spray bottle, about 1serum:2water, and spray it on to the ends and the lower half of her hair, as well as onto the brush.
Dd has just got the hang of washing her hair by herself, standing up in the shower - hooray! Still working on brushing and styling - she can do a couple of ponytail styles, but not plaits. I had to teach her to draw the brush right out at the ends, and follow it with the palm of the other hand, so as to make sure she didn't create tangles by lifting the ends with the brush and brushing them hack in at the top. It seemed obvious to me, but was not to her.
Last spring dd wanted to chop her hair off to a bob. I agreed, but asked her to think about it for a few months, because it would be a huge change and no going back. She has changed her mind. It's staying long.