Hi Syndey,
If you have an employee in your home as you probably know you would need employers liability insurance for Nannying purposes as you have a responsibility to provide a safe working environment. Now, some home insurances have this within their policy. If you haven't done this already, contact your home insurance as they will need to be notified of an employee working within your residence. Also, you can discuss with your home insurance how this would work in relation to another child being in your home who is being looked after by your paid Nanny. They might be able to advise you on the legal side of Nanny Shares.
However on saying this, Insurances such as NannyInsure (for the Nanny to take out) cover the Nanny for the care of up to 6 children (this can be from multiple families). This would cover your Nanny for any issues resulting from her care, it would not cover any claims arising from the other families child having an accident within your home (e.g tripping up on a piece of loose carpet and braking their arm/faulty electrical wires etc). Also, she would need to make sure that she notifies her insurance of any new children she is caring for including their medical conditions, allergies etc etc.
As for employment - if you accept the wages from another family and pay your nanny, I believe you would be classed as an employment business. As an employment business you are responsible for the Nanny's NI/Tax, Statutory Sick Pay, Statutory Maternity and Holiday Pay for both positions!! I would personally say that it would give you a lot of legal responsibility but it might be worth looking into a little more...
To avoid the worries of this, you could ask an employment business (like The Nanny Tree) to employ her on your (and the other families) behalf which means she would have one employer who would be responsible for NI/Tax & all legal and Statutory payments.
It's a mine field out there so I hope I could provide some clarity for you!! Good Luck!!
Chelsea x