DON'T GIVE UP! I have 2 friends who were worried about finding a job with their own baby and they both now have great nanny jobs. One has a baby the same age as hers 5 days a week, the other has two children plus her own 3 days a week.
One nanny I know takes her daughter to work with her half of the week and still gets paid a full salary. Some employers see it as advantage. You just got to look for the right ones. Horses for courses
You should however expect to be paid a little less than if you weren't to take your own child, but it is not a share, (unless you want it to be? ) If it's a share then the nanny has the right to say what she will and won't do, what activities she will be happy to do and pay for, she would spend half the month at her own house doing washing and cooking and not at the families house. That is how a share would work!