That's not inherently linked to the children being black, it's more linked to Black people being significantly more poor, as a result have way less access to swimming pools or vacations on water bodies, and therefore drown more due to lack of training.
I didn’t say it was inherent in being black, but if you are selling a drowning prevention technology, it’s imperative that it work well for dark-skinned people. As someone with dark skin I can say that the problem of tech products (Automatic red eye reduction, “Facetune” style image tuning, Face recognition login) not working as well for us is very real.
There is indeed a significant disparity according to https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6319a2.htm although that study doesn't analyze what may cause it (other than differences in the prevalence of swimming training).
I've always heard that the reason for this is that black bodies are denser than white bodies. Looking it up, it doesn't seem to be a huge difference though[1].