Fishing requires attention... I would suggest splitting your attention between a 5 yr old and your float is very bad idea. I saw a guy drag a kid around the old ranger run, the kid was entertaining himself (forced to) as the dad fished. The kid fell in a spawning channel and got soaked...he was not in danger yet the dad yelled at him...I felt sorry for the kid...a river is not a good place to babysit.
I have 2 sons - 6 and 7 yr old, I think they still are too young for most river fishing situations.
i have taken them out one at a time down to a pool with salmon and done some spin casting. even if there aren't any bites the cast/retieve thing keeps them busy and entertained. I actuall have them casting the rod and I simply supervise.
Taking a 5 year old float fishing for steelehad seems like a bad idea all round...they can't cast the typical steelhead rod, there aren't as many fish compared to salmon so boardom is an issue and winter flows right now are high. i don't know if your "scouting" means more walking than fishing but the chehalis usually requires bushwhacking, no fun for a five yr old either.