If I see a mum or dad struggling at the shops with a little kid, sometimes I poke my nose in and say something to the kid.
I judge each situation on its merits as sometimes it isn't appropriate.
Normally I just say, " listen to your mother" or a variation of that theme.
It breaks the kids concentration and a ugly mug like mine popping in out of nowhere usually has the kid wanting mums protection which stops the tantrum.
A smile and a wink to mum and quickly walk off, it's worked so far and haven't been abused from a parent, yet!
