Performance Bike has kids shorts & jerseys (at least in their stores). My 6 year old has a pair of Performance brand riding shorts. I found some kids-sized Fox full finger gloves awhile back (Bikewagon??) and bought 2 pair. Nashbar used to carry kids clothing as well.
REI also has kids cycling clothing (mostly in store versus online).
Speak to your local BMX club/shop as they do quite a lot of small sizes for kids aged from 4 upwards.
You can also look at Strider as they've started selling some stuff but its mainly rebranded which you can source via google.