“Almost all nannies start with their families — the families that employ them — within three months of a child’s birth,” said Susan Tokayer, a veteran social worker and co-president of INA. “So there is a steady supply, if you will, of new clients. And I would say the Boston area is an excellent example of a rich, fertile, market for nannies,” thanks in part to a young, well-educated populace and an array of local industries.