I was doing it at 10 with minimal supervision. the occasional yowch was a trainer supreme for flailing fails. I have no permanent scars.
supervision consisting of occasionally burning something up in a carefully controlled manner is needed. some kids take to tools quickly, some become "golden screwdriver" operators. how I would do it if I had kids is when junior came over with "whatcha doin?" I would exaggerate my procedure and do it slowly as a demonstration. when I got to a noncritical area, I would sneak the little nipper in by holding a heat sink, or the solder. eventually get a little gutless noncritical project started and let him have a couple licks at it.
remember, rosin is an irritant, molten metal hurts, and lead is a hazard. this would be a good time to get a little air cleaner on the bench, intake towards the soldering station.