I doubt Ofsted would be able to answer that one... it's a tricky part of the legislation.
When a nanny works for P1 and P2 it is ok, but not if a P3 is involved. But does the nanny themselves count as P3 if that nanny brings their own child... tricky.
But why would you want a job where you are caring for your child, P1's children and P2's children all at the same time? Why not look at jobs where it is only your child plus P1's children?
As a nanny you can't EVER care for children at your own home. Ok so that isn't quite true, but when putting it simply it is. (There is a 2-hour rule plus a 14-day notification rule, if you want to get more technical on this. )
So my advice is:
Don't care for children at your own home.
Nanny for only one family at any given time - this means that you can nanny for multiple families such as 2 part-time jobs, just not two families at the same time.
I doubt that there would be all that much of a problem doing a nannyshare plus taking your own child... but I am not aware of that legal point being tested at tribunal/court as yet, so if you do decide to do a nannyshare you need to be aware that there is an element of risk that you may inadvertently breach English childcare legislation. Legislation: The Childcare (Voluntary Registration) Regulations 2007 No. 730
The Childcare (Exemptions from Registration) Order 2008
a child or children for particular parents (?the first parents?) and, in addition, for a child or children for different parents (?the second parents?), wholly or mainly in the home of the first parents or the second parents or in both homes;
~~~ End Quote ~~~