H.
Your description at the end about her being ok until noise reaches a certain level, perhaps it is more about the sound than the kids. Perhaps even more specifically the range of the loud sound. Little kids tend to have pretty high sounding voices and screetches which only become more so when they are louder. If her ears are very sensitive it might be painful to her. Perhaps check with her ped about any thoughts and see if they might even want to do a hearing test to see if that might shed some light.