My son is 8 and can ride a bike but still gets very nervous on one.  He has a Rat Fink, which is a fat tire chopper style bike.  I actually think this might be making it tougher for him to ride vs. a more traditional bike.  I asked him if he would mtn bike with me if I got him a bike with gears, a front fork, etc.  He said he would if he could wear a full face helmet and body armor.    So now I am searching for that plus a kids bike that can handle the trails.  This event may be perfect from him assuming there is no prerequisite that they know how to trail ride?