Dandruff can be the result of different factors or a combination of different factors, so you may need to assess and rule out which one is relevant for your 8 yo.
I've read that dandruff is simply an overenthusiastic turnover of skin cells
on the scalp. Everybody's scalp sheds, but if a child has dandruff, the skin
sheds too quickly and too much. They say it's not common in kids until they hit puberty, but it sounds like your daughter may be starting early and may need to start using adult shampoos that will actually wash off the shedding skin cells better than the baby shampoos.
Also, if your child is like mine and washes her own hair, she may be using too much shampoo and not rinsing well enough. (The dried shampoo can come off in flakes that look like dandruff). Show her how to use only enough shampoo to clean her hair, and tell her to spend twice as much time rinsing it out as she does lathering it in, especially working it out of her scalp. Since her hair is thick, she probably needs to spend even more time trying to get the shampoo off of her scalp than usual. I've found this to be my daughter's problem.
My own experience with dandruff is different, so I'll share this with you, in case it's relevant. I have found that I'm allergic to citric acid, and used to have an awful case of dandruff until I took that ingredient out of my shampoos and conditioner. I still had a milder case of it every now and then after that, so I started using Pantene Pro-V Anti-Dandruff which worked really well and is gentle enough to use daily...until they added citric acid, so I had to look for something else... I found T-Sal (no citric acid), not T-Gel (b/c it has coal tar, which is found to possibly be linked to cancer). The directions say to use it 2x/week, so I used it 2x for the first week, which worked wonders on the 2nd application. And now, I just use it whenever I feel that slight tingling in my scalp that signals a bout of dandruff may be coming on, which may be 1x/month. It works that well. It is a bit expensive, so I only use it on my scalp and use my regular shampoo on the rest of my hair. But the cost is well worth it. It also does lather quite a bit, so you don't need to use too much, and need to rinse well. I know quite a few people who use it, and have recommended it to a lot of friends who now swear by it.
If home treatments don't work, you might need to consult a physician to rule out other conditions like eczema, psoriasis or ringworm.