I took my kids when they were 3 and 4 and they had an absolute blast. Best piece of advise I got was put them in rainboots and it worked fabulous. The rainboots plus nice wool socks and they were nice and warm. The bad side of the Smith is that you must be prepared for anything - snow, rain, sunshine cuz it can happen all in one day. Wish I could go with you, but we launch on Deso with our kids on June 3rd so we'll be on our way to Utah.
We fixed up a gear floor in the bow so that they were off the self bailing floor and could nap there under an umbrella - worked great. They loved watching the wildlife, fishing with Dad and running around camp. We took one of those ice cream balls and they had a blast playing and eating that!.
They've been down the Smith twice and loved it immensely. Hope you can find some other families. It's a great kids trip. It's great for adults too! PM me if you have any questions. We've been down the river multiple times with and without kids.
If you lean over to watch the river slipping slowly away beneath you, you will suddenly know everything there is to be known. Winnie the Pooh