First of all, thank you for the amazing and great advice.
I realized, after I had some more time to think about it, that as the "mom" and a primary care giver, it actually was my fault, in a way, that Miss Q got her head bonked because I did not appropriately train the babysitters for Miss Q (or for the dogs, for that matter).
So, understanding that our girls love these sitters and knowing that they (the sitters) are basically reliable and responsible (case in point, they did tell me immediately about the baby so I wasn't left wondering, "How did you get this goose egg on your head?" after I'd taken them home), we decided we'd definitely still keep them...if they even want to come back over to babysit after THEIR dog biting ordeal. Sheesh. However, I *DO* think I need to do a little more training with them about the baby...OR she needs to be a little older and steadier on her feet. I think everyone would feel better about that.
Again, thank you for your great pearls of wisdom. Even my mom reminded me, "Hey! You didn't know everything when Miss A was born (almost 7 years ago! Gulp!). And these babysitters don't even have kids of their own. Cut them some slack."
But, just as a heads up...remind your sitters (if you have small crawling children or offspring who babysit) that crawling babies are SQUIRRELLY and should not be placed on things they can fall off of...I'm just saying.