If someone is unhappy, it's actually really good that you know about it. You have the opportunity to learn and remedy the situation if possible.
Always reply to a negative review, and it's helpful to start by thanking the person for taking time to write a review. Parents don't just consider other parents' reviews; they also want to see your grace, tact, and responsiveness in a difficult situation. This is actually an opportunity for you to demonstrate your excellent customer service skills. It's also a good reminder that you should be asking all your customers for feedback all the time. If you're doing a good job, and if your customers are writing reviews, your positive reviews will outweigh the negative ones.
For more advice on handling negative reviews, see this article and interview of a Yelp Business Expert.