Lower Deschutes may be the only option for a multiday with Kids in Oregon this year. McKenzie is a good river, but its car camping territory.
August will still be brutally hot on the Deschutes, and there isn't a ton of shade. There are some pretty nice campsites that DO have shade, you just need to find them. A lot of guides have bad BETA on this stretch due to recent fires. That being said...
The Campsite just downstream from Washout rapid is the best on the trip - especially with kids. It is in a huge eddy at the base of a Class III rapid, it has a large "shelf" where the water is only 6-12 inches deep so the kids can play pretty safely.
The train runs along this stretch, but aside from that you aren't likely to see anyone else out there. There are some snakes. There can be bugs.
I've never done section 3, but if you launch at section 4 you can make it here in a single (long) day and layover if you want to. only problem is that if its taken (there are room for 2-3 parties here), you are in a bad spot.
but can you see its opposition | comes a-rising up sometimes | that it's dreadful and position | comes blacking in my mind | ....and then I see a darkness