They’d be intrigued, but also very wary!
If Sam ran into the smaller brother first, they’d both be so confused by finding another person their size around. Then if the taller brother comes in, Dean might try his best to get between the little guys and the other kid, resulting in two humans bristling at each other while Sam and the little guy realized what was really going on.
“Hey, you know him?”
Confusion ensues while Sam talks Dean down from his overprotective side.
Brothers Apart 