The G-5 and A-3 visa holders can hold a variety of domestic positions, including cook, butler, valet, maid, housekeeper, governess, janitor, laundress, caretaker, handyman, gardener, groom, chauffeur, babysitter, or companion to the aged or infirmed.
In general the G visas are issued to individuals employed directly by an international organization, or representing a foreign government to international organizations. 'A' visas are issued to representatives of a foreign government traveling to the U.S. to engage in official activities for that government.
No, your nephew or niece may not accompany you on G-5 visa. They must obtain the appropriate visa for the purpose of travel.