Probably because white kids live in sleepier more prosperous towns than black kids.
If your town has a couple of shootouts and driveby shootings each night, well then minor emergencies get cursory treatment.
If your town never sees a shot fired in anger and barely has a traffic light, well, they’ve got to do something in that hospital... so...
Could also be that they find out that the white kid actually has health insurance, and so they can arrange some lucrative tests.
Initially it's difficult to differentiate from failure to thrive (in the very young), Chron's disease, etc. and so forth.
That one condition could readily account for virtually 100% of the apparant disparity.