Is that a 20"? My boy is on a 20" from the age of 7, and will last him until 9.
http://earlyrider.com/collections/all-bikes are nice if you want new, but second hand is the way to go for kids bikes IMO. At least if you buy new, you know there is good resale on kids bikes.
EDIT: Just seen this:
As an example of their original approach, rather than numbering their kids' bikes by wheel size, Frog size their bikes by inside leg measurement. Hence the Frog 52 is recommended for children with a minimum inside leg measurement of 52cm (20.5 inches), so it fits children from 5/6 years old. It comes with a long aluminium seatpost with a quick release seat binder to make it easy to adjust the saddle as the child grows all the way up to 8 or 9 years old.
So a 29er for kids then.