How would you respond?

Although being a pro-baller is an unlikely occupation for most kids, kids from a low socio-economic background have no worse a chance at it than kids from a high socio-economic background. In fact, looking at the NBA, they actually have a BETTER shot at it than kids from a high socio-ecomic background (which still doesn't make it likely). Rather than shooting down those hoop dreams, encourage them to both study hard and play ball hard. Sports scholarships do get a few kids into a college that they otherwise could not afford or possibly even get into. Even many kids who will never get into pro-sports will be able to go to an otherwise unreachable college on a sports scholarship if they work hard enough and are talented enough. A sports scholarship into college is far more attainable than playing pro and it's not a dream you should shoot down.

In fact, don't shoot down ANY dreams. Unless you want to go down in history as that teacher who said "you'll never be able to do it" and was famously wrong. Although you'll go down in history just as "a teacher" rather than having anybody remember your name. Nobody remembers the name of Einstein's teacher who said he'd never amount to anythig. But everybody remembers that he said it.