My mother's trick for dealing with crying babies in stores:

[hedgehog]

If you see a baby or toddler whining or fussing, catch their eye and then give an exaggerated startle at their noise, followed by a sympathetic face. Something about interacting with a stranger calms little kids right down. I do this when i see a harried parent in the grocery store and it works. My kids call me the "baby whisperer".

A look of sympathy to the parent and a comment"Boy, I've been there!" sometimes helps, too.