Could this be because you are paying the nanny a flat fee per hour? Your 60/40 split suggests you are just working out the total weekly amount then splitting it, as opposed to it being hourly per family. That could be the reason why. If you pay half hourly rate while in the share so that each family pays equally, then pay a higher hourly rate when not in the share, it should be fine. So for eg, each family pays ?9ph while in the share, ?12ph when it's just you child. However, I'm not a financial expert!