Daryl only pulled his crossbow because he wasn't sure who was back there. After their run-ins with The Governor, he doesn't trust easily and everyone is a danger until he deems them not to be. When the kid comes out, one that couldn't have been much older than Sophia when she'd died, he lowers his crossbow and stares at him.

What in the hell was a kid doing out here?

"Are ya alone?" he asks.