I've seen them at Dusnkeswell and White Waltham, and I've flown them or given them a tour of the aeroplane, sitting them in it and letting them manipulate the controls while I explain what they do.
You can tell the keeners from the spotters because the spotters tend to have cameras and frequently they're not even interested in flying so much as collecting registrations. The keeners on the other hand have that look about them that they're seeing something really special, you know, the look that you had when you were looking at aeroplanes and dreaming of flying when you were a kid.
If you're a safe and competent pilot and there is a parent around to ask, take that kid for a 20 minute hop and you could be changing a life. And for pete's sake, give them some time at the controls if they start to look like they're enjoying it.