Taylor in the picture above is riding the small wheel version and the big wheel version in the picture below.
Keep in mind that if you do a conversion that you can do this as the rider grows.
Small wheel bikes are cheaper but you can always buy a big wheel or Expert later and then sell it as a small wheel. Its cost effective way of upgrading.
Top picture is at Thumper X at Currum Downs, bottom picture is at Thumper X at Packenham. Photos by Stephen Piper, Photographer, Upwey, Victoria.