Oh, I'd definitely wait to get out here before purchase. There are a couple of good bike shops in Oly, but for the widest selection of Dutch style bikes I'd hit Seattle or Portland for bike shopping. A bonus if you buy your bike in Portland before becoming a WA resident is that there's no sales tax. I wouldn't be surprised if there was an entire store dedicated to that style of bike in Portland. I swear I've heard of one, can't recall the name, though.
I'm not aware of any area-specific kid passenger or bike laws or difficulties. A lot of folks around here tote their kids on the back of Big Dummies or similar bikes. They rig up railings for them and load 'em up. I see parents with multiple kids loaded up in Bakfiets' cargo holds also. Those are nice because the cargo hold is in front of you so you can keep an eye on the youngin. Pretty much need a garage to own one of them though because getting them up stairs or through doorways is a chore.
A-Ha! Clever Cycles is the place I was thinking of in Portland. They carry Work Cycles, Linus Bikes, Big Dummies, an array of cargo and work bikes. I'm sure Seattle has a similar joint but doubt Oly or Tacoma do.
http://clevercycles.com/