I've seen several of the "Make and Take" events that Airfix have organised over the years - the format is simple, kids get a free kit, some paints, glue and tools, and are just left to it. Sometimes their mum or dad helps them out, sometimes they just steam ahead. Kids do not make models for the sake reasons as enthusiasts, they are not aiming to make an exacting replica that will stand in a display cabinet, quite often they're not even following the paint schemes but are making up their own!
At the Yeovilton air show this year, not only was the marquee bursting with kids trying their hand at a P-40 or a Skyhawk, but after they had deposited their model in their custom cardboard aircraft-hangar they were queuing to buy more models.
I think the trick is to just let them get on with something and only offer advice when they ask for it.