That's very interesting! It must have made quite an impression on you that you remember the incident all this time later - it must have been a very unusual powerful feeling as you say. This is pure speculation, but you say you'd had a few drinks. I'm not accusing you of being drunk! I just wonder if even a little bit of alcohol would sort of soften the usual input you'd get from the rational bit of the brain ("don't be silly nycuk, they're just going behind a hedge") and let the more intuitive primitive survival-y parts of your brain get their say across ("get over there nycuk, your wife and child are about to be eaten by a sabre toothed tiger"), which you were more receptive to acting on.
When you think about the scene, do you think it was something about what you were looking at that made you think 'there's something not right, it looks dangerous' (as they were going out of sight?) or did it feel more like a bolt from the blue (that you would have thought it whether you were watching them or not)? Impossible questions I know
I mean who knows, if your life were a science-fiction film, your swift action might have prevented some unpleasant disaster and nudged your time-line into a safe outcome, and you got a little premonition that echoed back and helped you! You say it still troubles you sometimes now, what makes you feel that? (is that because you worry what would have happened if you hadn't run over there? or because you don't understand what the feeling was or where it came from? or something else?) And why have you never mentioned it to your daughter?
questions questions, sorry