I highly discourage using any of those shock collars. They are especially bad when you're not home. What happens if one of your other dogs barks? Poor Sarah will be shocked for it. Then she'll start associating them and their barks with a very negative experience and that could cause fights. No matter how low the setting, I think they are not a nice thing to do to a dog, but especially such a little girl. Put the money towards a visit from a behaviorist who can help you solve the problem in a much more positive way.

This link has some helpful info:
http://www.ddfl.org/behavior/Barking.pdf