Often, when one member of a a child ensemble cast is sick, they are all sick. It is probable that most of the girls were also ill, creating a less energetic performance, and less quality vocals than their usual high quality. It is very difficult to perform sick for a child actor, when they should be home in bed. This is another consequence of this tough business on young actors and their families. When you are under contract, the show must go on.It can be very stressful for all.
But this is one reason that child ensemble shows receive less than gracious reviews after being favorably reviewed elsewhere on the tour. We forget that children are human and deserve time to rest and get well.