Does a baby red ear slider need gravel in their tank?

Not necessarily, but if you don't want to see a bunch of poop at the bottom of the tank, I recommend that you out gravel. Don't use the colored rocks. Those are painted and release paint over time. Investment your turtle gets bigger I recommend that you switch to bigger gravel so they don't eat too many of the rocks (they eat the rocks... its weird, mine have been doing it for years, and I can't find an answer to it.) .