"Upgrade Kit" for a 20in bike. (The bike in the picture shows how my girl's bike looked with the parts outfitted on her Specialized Hot Rock 20).

The fork is a Spinner 300 20in fork.
Air spring and lockout.
The forks are modified to increase the air spring volume, stock volume is so small you have to run the air pressure high and the kid barely gets any travel. The air shaft was shortened, lighter weight negative coil was installed. My girl was able to use full travel as a result. Additional springs are attached to fine tune for your kid if you so desire.

Front wheel is a Sun Envy 20in rim (super light and decently wide without brake track) laced to a 24h Novatec hub (lightest Novatec hub in their catalog). Magura Marta SL rotor.

Rear wheel is a Sun Envy 36h rim (with brake track) laced to a Nexus 7sp hub with a 23T cog installed (largest cog for lowest gear range possible for the kiddo). Laced with minimum amount of spokes since these are BMX rims, designed to land on concrete from rooftops after all and the 6-8yo is not going to generate that kind of force; plus they are to be used with V-brake.

Tires are Michelin Mambo; awesome tire with tons of trip on the dirt and fast rolling characteristic on pavement. Very large volume to boot.

Also included is a mechanical disc brake with a Tektro lever (which allows for super short reach adjustment, ideal for little hands)