Don't worry about it!
Even if your child has a *major* meltdown in the shop - anyone who has kids has only complete sympathy, and anyone who doesn't can not possibly understand.
Being the father of at least 2 drama queens, seeing someone else's kid being grumpy, surly, obnoxious, overtired, nagging, yelling, crying, screaming, punching, chasing, dripping ice-cream, etc. etc. in public just allows me feel that my kids are are cool, and just like those of everybody else.
So every time you are worried your child is a public spectacle, just think that you're bravely making all the other parents happy!
cheers,
(Mr)Wibble