I am reluctant to let my five year old go on an upcoming rafting trip. If you decide to take him, get him a hat with a wide brim and get the sunscreen stick so you can easily reapply the sunscreen on his face and arms, etc. You could also look into getting him one of the shirts with the sun protection built in. Regular white t-shirts offer very little sun protection, especially when wet. Contact the local rafting services and see if there is a drop off point partway through the trip. You could let the others do the major portion, then join them for the last leg of the trip. Like you, I would be a little nervous for the safety factor and the sun exposure.