IIRC, the St. Louis Zoo operates under an Amusement Park license. The KC Zoo may be the same. That said, the STL Zoo is NOT posted and I have carried their multiple times as I refuse to go to that part of town, or to their parking lots, in an unarmed manner.
As to your question as to whether or not it is "legal", the answer is that it is not unlawful. If you have a valid CCW permit and you are carrying your concealed firearm in the zoo, even if they have signs posted on the entrance, it is not unlawful for you to do so. In fact, it isn't even an infraction to do so. It only becomes an infraction if you are somehow discovered and asked to leave, and you refuse to leave. Of course, if you are asked to leave and you refuse, it is unlawful whether you are carrying or not.
Only you can make the decision as to what you think is right for your personal safety and the safety of your family.