I can't agree enough with the suggestions others have made, particularly the comments about kids' blood sugar dropping more quickly. A good variety of snack options, offered more frequently than at home, and a couple of hydration options that are 'fun' to drink (juice boxes, camelpaks, etc) really makes a difference.
We took a cheap paintbrush and trowel and 'hunted for fossils' in the river banks -- great entertainment when our son needed a break from the boat.
Also - crocs worked great for boat shoes. When they get chucked out of the boat, they float. And in camp, they weren't wet and chaffing his feet.
We used lightweight rainpants over the top of a wetsuit as splash gear -- they kept the wetsuit dry so he was less likely to get cold. And (probably not officially approved but...) we always have him in a helmet, but couldn't find a river helmet small enough, so use a bike helmet. It has a little brim to shade the eyes, and then too it protects not only from river rocks but also from whacking his noggin on the drybox or pelican cases if he's hopping around in the boat and slips.
We also take a giant golf umbrella or small patio umbrella. Not sure where you are, but we get occasional downpours even in the middle of summer. Several times it has saved the day. And it's endless fun for him to hide under the umbrella with one parent and giggle like mad while the other parent rows in the rain
Other basics: sun hat, long sleeve rash guard/swim top to prevent sunburn, lots of headlamp batteries, and brush up on your camp songs.
Sounds like great times in store for your family!