OK, so this was a film from at least the early 80s, probably the 70s, saw it when I was a kid.  I think it was British-made, it may have been a TV movie, not sure.

Anyway, there’s these 2 kids (can’t remember if it was 2 boys or a boy & a girl) and there’s a guy who is trying to kill them for some reason, and attempts to do so on more than one occasion.  At one point, he takes them up to a cliff overlooking the ocean, and tries to kill them, but they thwart him and send him over the cliff instead.  I seem to remember he was wearing a disguise of some kind, which is why they didn’t recognize him.  I remember there being some kind of car, a roadster I think they figured in a scene.