I find that type of signeage misleading too...although I'm sure plenty of people here will argue with you.

That said, I do think that as a parent, you could have shown some spine and said to the cashier, "Oh my mistake. I assumed that $19.99 was for the set. In that case, I'll put these back."

Nothing wrong with owning up to a mistake and letting your daughter see that you have the fortitude to say where and when your money will be spent.I think it would have been a good lesson for her.

You say you didn't have 'the heart' to disappoint her. That's too bad, because really, letting kids see how the world operates and how to gracefully handle disappointment is part of your job as a parent. So maybe next time don't back down.