Some dogs have not had good experiences with kids. Or have never been socialized with them. This can cause them to be nervous or not tolerant of having kids around. Also some dogs are just big & boisterous & could injure kids due to their level of excitability. All of our dogs go through a series of temperament testing here at the shelter. The dogs in this category have shown signs of having lower thresholds which smaller kids would not understand as well as adults can.