A CCR won't plane with your typical trolling motor.  So small fry wouldn't be at too much risk at idle speeds.  The CCR's air tanks provide much more floatation than needed to keep the hull afloat if the cockpit were swamped.  And the side decks keep occupants centered in the hull, minimizing a capsize potential (I've seen adults comfortably sit on the coaming/side decks with their feet in the cockpit).  There are still safety issues with small children in small powered craft that parents have to consider.  Also, there might be state licensing requirements.

All that aside, if you can build a CCR (not difficult), you're certainly capable of figuring out how to clamp a trolling motor onto the transom and put a battery into the cockpit.  A step up later would be to use a 2- to 4-hp outboard.  Although I warn you, I've seen an adult drive a CCR with a recent 4-hp, 4-stroke outboard at 15-16 mph. YMMV  Good luck if go for this (as a kid I would have been in heaven with a CCR and just a paddle).

I like your proposal.  With proper motor selection, shaft speed and propeller pitch, LiPO batteries and an inboard arrangement with some solar panels for recharging, it would work.  Keep us posted.   Trolling motors won't cut it.  

How young? The APBA has a Junior class runabout & hydro where kids can drive at age 9. It is "restrictor" plate racing with a carb restrictor plate on an OMC A racing engine that keeps speed down to around 45 mph. 

I don't know what each state's or each country's laws are with respect to licensing of drivers of electric boats.  Some of the younger racers don't need boating licenses in some sanctioned races, although they should be licensed for practicing in their home state if local laws so decree.  The APBA's required instruction/testing of young drivers is probably more intense in some areas than state tests.  My sons obtained their state boating licenses at age 10.  If we had had an electric powered CCR then I would have let them reasonably loose in it (mom might have jumped down my throat, tho).  I most certainly wouldn't have let them loose in a boat that can go more than twice as fast as a 6-hp powered CCR.

Those youngsters that do race in APBA's runabout and hydro classes are watched over carefully.  More power to 'em.  I've been jealous of them for decades.