Well, being a member of the royal family is a job. If she's on that balcony, or attending events or whatever then she is, in fact, working. She's doing her job. All mother's who are working and in the middle of their working day, need to put aside their children during that time. If an emergency happened, you'd get a call and you'd deal with it. A baby crying isn't an emergency and she was working. They have nannies, so she has it easier than most.
If it wasn't anything to do with the baby crying, then she needed to be told to turn around because her job required it.