Before escalating the situation, review your nanny employment contract. Does it clearly spell out the nanny’s responsibilities? If not, there may be a simple misunderstanding. If that’s the case, raise the issue with your nanny and discuss whether the employment contract should be amended to clear things up. Put any amendment in writing, and be sure that both you and the nanny sign it.
If the job duties are clear in the contract, look at what the consequences are for failing to fulfill those duties. Are you required to issue a warning? Or, instead, do you have grounds for firing the nanny? If you do have grounds, do you have to give prior notice?
Whatever the situation may be, follow your nanny contract. In addition, put warnings in writing and keep a copy of those documents. The writing can be phrased in a way that alerts the nanny of her failure, but that isn’t overly negative. For example, you could say: “[Nanny’s name], could you please remember to clean up the children’s room at the end of the day? It will save the time I spent on it yesterday, and I’d greatly appreciate it.”