Here's KatieB's easier question:
"How do I kindly tell fathers at preschool storytime that their daughters are a constant disruption to the stories and the librarian has to stop a number of times each book because they can't go on with the story? The dads sit out of the room reading while other parents and I attend to our children in the storytime room. I also have to work with one of the dads from time to time, but I never liked him even before his daughter was a brat."
I'm betting I'm not the only one laughing, because we all know someone we've disliked for years who, unsurprisingly, turns out bratty kids. To be forced into storytime with the bratty kid is adding insult to injury.
It's not your job to tell the fathers. They shouldn't be allowed to sit in the other room reading if their kids are being disruptive. (The library really should be on top of that, because depending on their insurance coverage they could be open to a huge lawsuit if something happens to an unattended kid.) And their kids shouldn't be in the room if they're being seriously and consistently disruptive.
After the storytime one day, approach the librarian and express your concerns. The librarian is surely pissed herself, but probably can't do anything about it unless she receives feedback from another parent. You might want to bring up the liability issues because that will put a little mustard behind your request. I think the librarian can fix the problem without causing major hurt feelings and without outing you as the narc:
1. She prints up signs that say "All children must be attended by a caregiver at all times" and posts them prominently in the storytime room and in the room where the dads lounge.
2. As the dads try to leave the room after dropping off their kids, she approaches them and says "We've been informed by our insurance company that a caregiver must be with each child at all times. I'm sure you understand" and then gives them a big, sweet "I'm a little librarian" smile as she steers them back into the room.
If the kids still act up, after the storytime is over she could approach each dad and say something about the kid "not seeming to be that into storytime." At this point, the dad should know that his hour of free babysitting has evaporated, and he will either step up to the plate to supervise or not bring the kid to storytime anymore.
But this is definitely not your responsibility to take care of. Let the librarian know that it's really disruptive to you, and if she seems at a loss to do anything, talk to the head librarian or manager about it. It's not right or safe for the kids to be disruptive.