I grew up on the Deschutes and was fortunate to include floating the river to my list of activities each summer. There are plenty of swimming holes that work well for families, but I don't recommend "floating" as a child-friendly activity. There are tricky areas to navigate and a nasty undercurrent. This is best saved for the adults.
Tumalo State Park is right outside of Bend Oregon. The Deschutes River gently meanders through it. There are places on the deschutes with class 4 rapids, this area is very calm, and shallow.
The kids can walk across the river in most places. There are picnic tables around and beautiful trees to lounge under with grass areas that go right to the bank of the River. The kids float down on regular inner tubes, get out, walk back up the river 1/4 mi. then do it again! It is a lot of fun and fills a day.
You do have to pay a park fee which is approx. $4 per car.