A plethora of answers and the movie itself reveal that children are in fact non-toxic for monsters.
In this scene, right after the conversation with Waternoose is concluded, Mike and Sully walk away. At this point a CDA worker points his/her geiger-counterish device to our heroes, and the device actually gives a warning, but a passerby gets jumped, and the scene ends.
So if the children are non-toxic, what kind of energy does this said device detect?