once, in a store, some kid started "playing" with my termies-whilst i was half-way through a game.
what i did was i asked him politly to please stop playing with my modles. solution.
Good for you, being polite. Depending how much 'playing' was going on, I would probably reacted from a calm, yet stern "Don't touch those" to a very aggravated "What the fuck do you think you are doing?" Slap the kid, deck the parent. Of course, here in the US the hobby tends to be more popular with adult males in their 20s. Soldiers are pretty common where I live. So the little shits... I mean 'children' tend to be a bit more respectful of our stuff.
Well, that is my $0.03 worth (damn you weak american dollar!)