The travel agent, whose window this was seen in, definitely wasn’t thinking when he put the sign up. Or hadn’t he read it properly?
Sometimes we know what we mean when we write something but it doesn’t mean someone else does. It’s like having part of a conversation in your head and then continuing it aloud; you know what you’re talking about and you presume others do too, but they have no idea.
The person who put this banner together knew that she/he meant that kids can go on holiday for £1 and upwards during the sale period but the wording actually says something quite different.