"In another context, you would scream “racist!” if someone called Brown or Martin “boy.” Yet you refer to them as children."

Well they both were very young, so yes. They were both boys, or young men. Terms that tend to be interchangeable in a general sense. But hey, grown men in their late teens - early 20's in the military are often (with some level of endearment) called boys or children. Sometimes they're even called girls. Before you throw in any more red herrings, let me just say it is all about CONTEXT. The historical context to which Whites referred to Black men as "boys" was racist.