How big of a kid? Budget? How much riding?//

Bike Friday makes the Bantam bike for those shorter folks.

https://www.bikefriday.com/betatest/bantams/

But, they aren't particularly cheap.

I would think some of those super-compact small bikes could also be adjusted small, for example the 12" mini-folder.
http://www.amazon.com/dp/B00LZO7KL2/

Or, perhaps one of these 12" bikes.
Portable 12 Inch Folding Bike Custom Mini Folding Bicycle - Buy Mini Folding Bicycle,12 Inch Folding Bike,Mini Cooper Folding Bike Bicycle Product on Alibaba.com
The new 12 inch 16 adult men and women children shock folding bike BMX bike Mini super light-in Bicycle Frame from Sports & Entertainment on Aliexpress.com | Alibaba Group