I would definitely tell her. If you are worried you could have an incentive ie. If she goes to bed and doesn't be silly she can do xyz the next day, or she can have a special breakfast etc And you'll be asking xyz if she was a good girl etc. I think of it from their perspective they think you're going out to have fun without them and at the same time they are probably very excited at the prospect of being babysat. So if there is an incentive for good behaviour for the first few times they start to realise it's no different from any other night. We did this with both DD & DS when they were little, and occasionally if we are doing something out that they feel they will miss out on we'll do it then too.