There is no reason to discriminate against foreign-born nannies who speak good English.
Furthermore, those who take care of children professionally understand that they are being paid to raise the children according to the rules of the household. The only issue is that you may not want to let them prepare food, because they could treif up the kitchen. Otherwise, I don’t see the problem.
In families that have two full-time breadwinners, some sort of childcare is a necessity, and not everyone is lucky enough to have healthy parents living nearby who can babysit the grandchildren. It’s not a matter of how many children you have, it’s just that a full-time work schedule doesn’t always sync with school schedules (plus, what do you do before they start school?)