Nanny rate rise for sibling

[sheepi]

Hello! We have a long term part time nanny who looks after our DC1 3yo a couple of days per week and has done since I went back to work after maternity. Our nanny is happy to do some hours looking after DC2 as well who is 6mo. What would be an appropriate rate increase for those hours where she has both kids? Would only be for 6-8 hours per week at first so I can get back on top of things and transition DC2 into knowing her before I go back to work. We live in London and charge £19/hr gross (£15.50/hr net) for looking after DC1 only as things stand.

I've always experienced nannies to get paid to be a nanny rather than be in paid according to the number of children n their charge( excepting looking after multiple births as they can be paid higher anyway).
From my experience (in London) 15.50/hr is a low wage no matter how many children it's for so id be increasing based on that not the number of children in the nanny's care.