Rappers Chris Smith and Chris Kelly of Kris Kross may be only 13 but don’t call them cute. Not your typical kiddie act, they talk tough, try to spark fashion trends, and do club shows at 2 a.m. (What happened to child-labor laws?) The bottom line is that their smash debut album, Totally Krossed Out (which reached No. 3 its second week on the charts) aims directly for your bottom end; the full-bodied musical track thumps and swings. Any hip-hop fan will have heard these beats elsewhere, but they’re just as funky the second (or third) time around. Play the group’s hyperactive platinum single ”Jump” at any party and watch the floors quake. To their credit, the two rappers don’t rely on their production team’s musical prowess. Smith (who calls himself Daddy Mack) and Kelly (Mack Daddy) grip their microphones with so much confidence that if they didn’t sound so youthful, you might forget they’re just barely out of grade school. They avoid bubble gum and playground themes; instead they rap about sneaking into clubs, picking up girls, and having nightmares about growing up to become gang leaders. If Kris Kross continues to be this fun and funky, neither Smith nor Kelly should worry about life beyond childhood — they’re good enough to keep making hits well past puberty.