I don't see the problem with reaching out to the parents via message. I don't know why some parents here are saying a direct in-person approach is less creepy, because I feel the opposite. You can ignore a Facebook message, but you can't ignore a stranger that is in your face. You may also be in a hurry and don't want to deal with all the in-person questions, whereas on a Facebook message, you can respond at your convenience, night or day. Besides, I don't like strangers coming up to me and questioning me, period. A stranger trying to make conversation in person already makes me wary, and body language and appearance can make that experience even more awkward. I'm just a shy person I guess.
I am on Yelp, a site used for writing reviews of businesses, and I get people messaging me all the time asking what dishes I recommend at a restaurant, how my experience was, etc., if they want more information than what I mentioned in my review. Some people have empathized when I had a bad situation at a business and shared their own bad experience.
Why not give it a try? The worst that can happen is they ignore your messages, the best that can happen is they respond honestly, without bias (which may not be the case with school reviews, which are sometimes monitored by the school and any negative comments removed), and you end up knowing a parents whose child attends the school and can give guidance/recommendations about how the school operates. Who knows, if they have kids the same age as yours, they may even end up in the same class and you and your child just made some new friends!