Daily costs of nanny share?

[sheepburn]

Hi all
I'm currently in a nanny share which has been running for 18 months, switching every two weeks between the two family homes. Each family has 1 DC but from next week my DC2 joins the share. As I will have 2 children in the share, we have decided to keep the share at my house every week. We have decided a split (£7 for us and £5 for the other family) however what is a reasonable amount I should ask the other family for, to contribute towards costs?
We currently give the nanny £30 kitty for 4 days and I anticipate this will be split by both families.
Your ideas and thoughts for additional daily costs (nappies, food, wear & etc) would be very helpful!

I have a nanny share that is based out of the other family's house.

I contribute £2 per meal and add a bit extra for snacks, send my own nappies, wipes and milk. We split the kitty and petrol expenses 50/50.

I don't pay extra for wear - the nanny does light nursery duties so the other family get the benefit of that. I don't know how you would quantify wear and I would feel a bit odd to be asked to contribute to this. After all your DC are getting looked after in their own home with the comforts that brings - own toys, own beds for naps, you don't have a childcare run etc.