The ferry price is only collected westbound to Friday Harbor on San Juan Island (or to Lopez, Shaw, Orcas) but both ways to/from Sydney.  So if you wanted to island hop you would go out to Friday Harbor then you could visit Orcas, Shaw and Lopez (in that order) on your way back to Anacortes for free.  (On your way back to the mainland, make sure you read the signs and get your car in the correct lane for your next stop or you won't get off where you want to.)

I'd stay another day in the San Juans instead of going on to Victoria.  Take a whale watching or kayak trip with the money you save on ferries.

A 12 year old girl would probably enjoy visiting the alpacas on San Juan:

"Honey Lane Farms Yarn Studio
289 Honey Lane
Friday Harbor, WA 98250
360-378-1895
FAX: 360-378-1895

Our Alpaca Farm and design studio features natural colors of Alpaca, 50 custom colors and hand painted Alpaca yarn. We produce our own knitting pattern kits, handmade soaps and weather sticks. Our Alpacas are eager to be fed from your hands. "

I assume you are already aware of Lime Kiln Park and the Whale Museum (very nice!).

If you stop on Orcas, Mt Constitution is fabulous if the weather is good.   Lots of waterfalls in Moran State Park if she likes to hike a little.  The Orcas Island Funhouse is a good place to go when the weather is bad (or even when it's good).  http://www.thefunhouse.org/about.html

Ron