Make sure you are aware of the space you have for the rug - I think that goes without saying, but kids rugs are a tough one. You want something that's easily kept clean - at least that is what I would want. Therefore, light colours such as white and cream would be a no go as they would quickly end up looking brown or grey.
I'd also be thinking about the design, and for how long it would be age appropriate. I think it also depends on how much you are willing to spend and/or how much time you want to have before you have to go out and buy another.
The light in this image is all wrong, but I quite like the idea.