Fun things to do that cover the 7-12 year old range and can accomodate 1 with a wheelchair while another burns off energy running circles and yet another cloud gazes.  Any ideas? 

--Main thing would be somewhere my son could wheel around easily with his wheelchair and we could all have fun.

Have you been to Randall Park to feed the ducks? They also have music there on Wed(?) nights during the summer. Take a picnic!

At Franklin Park there is music and the arts van on Thurs. nights.

The Greenway?

The museum?

Kids craft classes offered at Michael's, Allied Arts, Scrapbook Central.

The Library?

Kaner Flats Campground is paved.

We love the drive/grounds of Mary Hill Museum. We stop in Goldendale and pick up Subway then eat while we watch peacocks.

Northwest Trek is a great small zoo. Easy to get through in a day and pretty affordable. 

Best of luck! (It only gets worse. They turn 15 and EVERYTHING is dumb, stupid and, gasp... requires clothes that aren't sweats and getting up before noon!)