Little Paternalistic Lies of Convenience

Driving in this morning, listening to "Marketplace," the announcer recalled the story of a friend whose mother told him when he was a child that when the ice cream truck was playing music, it meant that it was out of ice cream.  Parents play on the ignorance of children with these little lies of convenience -- or are tempted to -- all the time.  Kids don't know how the world works, so they turn to us to construct it for them.  Sometimes we construct their world so that it does not match up with ours, but the constructed world makes theirs and our lives easier.  In this case, the mother not only doesn't have to hear constant pleas for ice cream (pleas as opposed to "please" which is often whined in pleas), but the child does not have to suffer constant hope and disappointment.  His life is better.  But it is a lie.  Is such a little paternalistic lie of convenience justified?