We had a tiny bit of this. Nanny was allowed to bring her son on days DCPS was closed. Her son was 8 to 10 during those years. Our DS adored her DS and her DS was old enough to read and was naturally good with kids, they would even play basketball together with our toddler hoop. On half days they would walk to pick up her DS at school. It was a major plus to the nanny and we considered it a positive thing as well. But I would not have done this full time or with a younger child.

Does the nanny have prior experience nannying for three children this young full time? That would be my greatest concern. Setting aside the issue of her bringing her own child, three under three is very challenging. We had a nanny share for a couple years and broke it up when one family had a second baby. We saw more downsides than benefits to one nanny with a newborn and two 2.5 year olds.

I just don’t see how this is different from anyone else asking to bring their kid to work with them. It changes the entire nature of the job and job description. What other job allows you to bring your child?