Miles Per Hour vs Bullets Per Kid

Okay, so here's the deal, genius. The joke is making a dark comparison between the United States and Europe. Americans measure speed in "miles per hour," but the joke suggests that Europe could be measured by "bullets per kid" implying gun violence affects kids a lot. It's morbid because it's talking about kids getting shot. And since you're reading this, I guess you didn't get it. Sad!