I suggest that your reaction to the little girl is quite normal. Seeing her cry, as you said, brought up those feelings for your son. You'll be less sensitive to this in a few day. I wouldn't worry about how you're feeling. Remind yourself that this isn't your child and you have a job to do. I bet you did that tonight, already.
You can learn how to turn off the worry. Think of a helpful and positive phrase to say when you start to worry. "This is not my son. My son is OK" said several times to yourself might help. Turn any negative thought into a positive one and repeat it while pushing the negative thought from your mind. It took time but I've learned how to turn my worrying around.
After your SWH I suggest that your reaction is more serious than one would expect. If you're not already seeing a therapist I urge you to start. You do not have to suffer this much.