I have had 2 experiences with Nannies who were mothers and brought their kids to my house while working. I thought of all the nice possibilities - my daughter who was 4, 2 years ago, would have a built in playmate and my acquaintance was able to give a bit of a discount as we are not rich . Her 5 year old son dominated my daughter, breaking her toys, even reaching over for her food when he had his own. He was angry, when his mom was doing her nanny job with my daughter he would sometimes have really disturbing tantrums. She needed the money and just acted like there was no problem at all. My boyfriend and I ( not married) did not feel comfortable leaving her at home.by herself, we were able to get both our mothers to help a little but finally we had to say no more - it was very hard on the acquaintance, we somehow pooled together some money for severance.
Fast forward- a year later we enlisted a different nanny/birthday specialist to bring the cake/supplies/games/decorations and much more and she regaled us how hew 3 year old daughter would dress as "tinker belle" and "help". Tinkerbelle tore up our garden - methodically killed all the flowers and vegetables. I am assuming because of anger seeing her mother make much of our daughter as the b-day girl.
I am sure there are many situations where a nanny/mom gets it right and I do feel badly for those in hardship but we had these 2 experiences