Hi,
I have bi-racial children, too, and here's what we do that works wonders. (You may have already tried this, but in case not, here goes!). First, we brush mainly when they are in the tub. After shampooing gently and then putting massive amounts of conditioner in their hair, I brush, while the conditioner is still in their hair. After rinsing and getting them out of the tub, while their hair is still wet, I work creamy baby oil into their hair and brush it before putting into braids, ponytail, or whatever (once their hair was long enough--before that, we just let it stay out and look beautiful and wild). The creamy baby oil works wonders and gives their hair a nice shine. Many places do not sell it, but we can usually find it at Walmart. Here's a link to the product. It is not expensive. http://www.johnsonsbaby.com/product.do?id=38
I know a white woman with a bi-racial daughter who was so obsessed with her daughter's hair being "unmanageable" that she gave her poor daughter a complex because she wanted hair like mommy's. When the daughter was in elementary school, she begged to have a hair cut like her mom's and to try to make it as much like it as possible. It didn't work, and her haircut and style looked terrible. Now, the girl is back to her "own" hair and wearing it in styles that suit her particular hair, and it is beautiful!
Best wishes finding something that works!