I've never had a problem letting the boys wear costumes in public when it isn't Halloween. They are only little for a short amount of time and if something as simple as allowing them to wear a costume to the store makes them happy, then so be it. I know there will be a day when the dress-up clothes are all packed away and donated so I will take this time to savor and enjoy it. I know random strangers get a kick out of it too.
Do your little ones like to dress up and do you allow them to wear costumes outside of your home?