We needed a temp nanny for 3 months recently whilst I did some work, we used an agency.
Had quite a few candidates to choose from, one of which was a nanny with her own child.
After meeting her (and the others), we decided we really liked her, and my dh and I sat and talked through all the pro's and con's. I admit I was a little worried about how things would work.
I was very pleasantly suprised at how smoothly everything went, and to be honest, it was a very good situation.
She was very organized, and brought along with her, a double buggy, extra booster seat, we had a travel cot anyway.
She said that she would bring all her childs own food etc, but I said no, that her child could share exactly what my ds had.
I never worried about her putting her child first, both children needed different attention at different times, and I do believe that having another child around, made my ds more comfortable about not having me around. He was happy to see nanny and her child every day, and he couldnt have been bothered about me leaving.
At one point her child was hitting and biting, and she felt awful, but these things happen, and I wasnt at all concerned. I felt confident leaving my ds with her, as she was a mum, and used her experience and intuition.
We didnt pay her any less, the usual rate.
I can highly recommend a nanny with her own child.