I think what you're asking is - would it bother you if the nanny / babysitter / child-minder nursed her own child in front of your children?
No, it wouldn't bother me that she would nurse her child in front of mine as long as she realizes that if my child is verbal, he would ask about it. She would need to be prepared to answer questions.
The other part of the question is unspoken. Nursing a baby can take up to 40 minutes every 3 to 4 hours, so if you are nursing a baby, that's a significant amount of time that you cannot tend to the other children in your care.
Yes, you can interrupt the nursing and have your baby start to cry because you have to take care of something that came up with the other kids in your care. But when you are babysitting my kids, my kids need to come first. That's why I'm paying you to watch them.