I think what I would do is when this child wakes in the afternoon, do not get her up. If you get her up then she doesnt have the opportunity to learn to self sooth and fall back to sleep. Nap time is also quiet time so if you have a 2 hour window that you require quiet from all your kiddies, then whether they are sleeping or not is irrelevent. They are still staying in their playpens or on their cots for quiet time. They still get to rest and you get your break also. If this little girl is being to disruptive then move her to another space where she won't disturb the others and let her self sooth. Whether she is whaling, sobbing, singing or talking, she needs to be left to learn to calm herself down. If she knows you will always cave in and get her, then you might not see her improve because she doesn't need to self sooth, you do it for her.

This can be hard on you if its difficult for you to switch off and ignore her. We all know the sound of a cry from a child that REALLY needs us, versus wanting attention from us. Sometimes this process is quick but it can also take a LONG time, but it has to be done. I would also try and get the parents on board as if they do the same at home then she will learn much quicker. If not, she will learn eventually what your rules are even at this young age. But the longer you leave it to enforce tough love to get her on your sleep schedule, the harder it will be for YOU.

Good Luck and the minute you feel like you might give in and get her up, come here and vent.