We've been seeing this black and white rug a lot lately for childrens' spaces. In fact, we just featured a room using this same rug yesterday.
For the most part we like to see something graphic for the floor in a kid's room but too many patterns in one small space can be overwhelming. The black and white stripes makes a great statement and grounds the room without looking chaotic. The slightly staggered stripes gives it more visual interest than conventional stripes. Get it at IKEA.