I personally would avoid a fixed blade for boys of this age. I would go with a folder first. This helps them to understand that they need to thing about using the knife before just whipping it out. I know there are dangers with folding it back up, but I think I like the idea of a non-exposed blade for carrying purposes. You may want to consider a SAK-type also. Kids like all the gizmos. Also, it will allow you to teach them the limitations of the tool.

My $.02 worth