Whenever I'm doing this with my kids i tend to show them videos.
This video, shows a man walking forwards (in a rather silly fashion...). When played forwards, which way does he go?
What would happen if it was played backwards?
This is a video of a (sadly now deceased) man walking backwards... If i play it forwards what way does he travel?
What would happen if i play the same video backwards? He moonwalks forwards!
So the first vid is walk forwards (+) * played forwards (+),
the second is walk fwd (+) * played backwards (-)
The third vid is walk backwards (-) * played forwards (+),
the last one is walk backwards (-) * played backwards (-)
This then leads to obvious questions like "right, MJ moonwalks 6 steps, how far and what way would he go if i play that vid backwards 5 times?" - then do it - play it back 5 times and count where the dude goes..
The nice thing about this is it allows me to cue massive moonwalk practise for my classes...