It's time to use adult shampoos that say "clarifying." That's what I had to start doing for my eldest daughter for the same reason. I had to teach her to wash only at the scalp and not through to the ends because when she rinses, the soap goes all the way through anyway. During the day I make sure she brushes her hair with a bristle brush (boar's hair) a couple of times to help distribute her hair's natural oils. I also let her skip a day of showering on weekends if we don't have anywhere to go. If her hair gets static, then she gets a very small amount of conditioner on the ends of her hair only.
My middle daughter has been having to use adult shampoo and conditioner for even longer than my eldest because her hair is very long, curly/wavy and very thick and heavy. She's had adult-like hair for a long time, so I need to treat it like adult hair. My hair is curly so I just use my products on her. Everything is extra moisturizing.
My youngest is only 7, and she uses a simple and basic adult shampoo and conditioner that's moisturizing because her hair is pin straight and very long.