when the bottom of your feet are black/brown/any other color from being barefoot and not washing them. a little kid may never get to scrubbing his feet in the bath, and they may remain dirt encrusted if this is the case. you can say the phrase with extra disdain if it has been 3+ days and the dirt is still there. but mostly if someone has been running around barefoot all day, they'll end up with little kid feet. and you know what? that's okay.
"yo, have you seen kiwi's feet today? he's got bad little kid feet"
"yeah, that's mad gross, but i'm not going to shun him. dude just likes to be barefoot."