I don't know how much it will help your little ones, but my 8 year old has a whistle on her vest for those "just in case" moments. If she finds herself in the river and floating away she knows she can blow the whistle and get our attention. Now before I get flamed....we watch her like a hawk when we are on the river, but it gives us a little peace of mind knowing she has a way to get peoples attention if in need.
My 8 year old and I have been doing high flow "Daddy-Daughter" trips for 5 years on Ruby and were actually interviewed by the Grand Junction News once at the boat ramp for launching at 49k (super high water). We have always had a great time even with the flooded out camp sites. Like many have said before me, unless you have a really good eddy, swimming or being near the water should always have a adult with PFD nearby.
Bring some other games to play as swim time for the kiddos will be limited. Horseshoes, Uno, Nerf ball, beach games and hikes have always allowed us to have a great time.
Many of the channels are open at high flow, so with a little caution...the channels can be great time to adventure on the float down. Left of Dog Island is about the only one we have not been able to get down without getting out and dragging a little.
Other than that....it still a great time with the kids. Just use a little more caution as the water is moving faster. AND as another person posted on here...if your staying at Black Rocks pull over early and go scout your landing for your site. We stayed at Black Rock 9 once when it was flowing 30k + and had lots of up river guest that missed their site. If you do miss your site there are some less desirable "oops we passed it" camping further down river.
Have fun and remember its not about the destination so much as it is about the journey with the kids.