The study I found said that it is a GOOD thing because the mixed kids will be smarter and taller.

This study found no such thing. I see it misrepresented by journalists all the time. I guess because they used term like "genetic diversity" it made them think of brown people. But no, this study has nothing to do with mixed race people. Judging by this comment

We think genetic diversity decreases the chances of inheriting defecting copies of the same gene from both father and mother,” said Peter Joshi, first author of the study, The Independent reported.

They just found that non-inbred people are taller and smarter than inbred people. Which is to be expected. Race mixing results in intermediacy of traits. Children will have IQ or height or whatever as is the mean of the races they are composed of.