It is always good to see another rider in Mill Creek. Your local options are:
The Bicycle Center across the Bothell-Everett Highway from Fred Meyer
Gregg's, REI and Performance, all down by Alderwood Mall
Bicycles West and Harvy's Bike Shop in Lynnwood
The potential problem is that for just about all shops, it seems, the selection of kid-sized bikes is low. Bicycles West in Lynnwood had a pretty good number of kid-sized bikes on the floor when I was there last month.