You need to get rid of the biracial term and start using mixed. By only using biracial you're leaving out people that are multiracial. There are people that are 1/4 black, 1/4 white and half Chinese for example that would be in the Mixed category. But by you only using biracial you're leaving them out.
I also disagree with you that a mixed person + black = non-mixed kids and vice versa. That grandparent just disappeared?
You're also going to try to tell me that these kids aren't mixed?
Their mother is mulatto and their father is a white man.
Nope those kids are growing up in a White household culturally and are going to marry White.
The multiracial population isn't big enough to consider of importance.