I've been a nanny for the past almost 6 years, for a family with 3 children. Their ages are currently 7, 12, and 14. When I had my son (who is now 3), they arranged other care for about 2 months so I could recover and get into a routine, then I came back. My pay was not lowered, and it didn't need to be. Yes, I was paying more/different attention to my child, but I wasn't giving the other children less attention. It's also much easier to incorporate another toddler (hers) into the group's activities, and I don't believe she would ever DREAM of expecting you to pay for her child's fare to anything. I also don't believe the parents should have to pay for an expensive activity that the nnny decided to take the children to. If I took the three kids to the zoo, because *I* wanted to take them, why should their parents pay for that? They didn't decide to take the kids. However, this isn't something I ever discussed with the parents, I just didn't charge them for activities I chose to take the kids to, nor did I charge them meals I paid out of the house for (unless the parents requested this meal/activity/etc for the children). After all, acivities are available at the home, for free, as is food.
Long answer short, I wouldn't dock her pay unless you see something amiss. She's a mom, more than likely she's good at juggling what needs to be done, and you (hopefully) won't find her job lacking. And if nothing else, your kids will benefit by having another playmate.