How do I split nanny share pay with new baby?

HoHoHo - I don't think you can have that both ways. Making the parents pay 'per child' for a nanny, but not paying a nanny per child, why should only one set of parents benefit out of this arrangement? In most instances if the family with one child had another child they wouldn't pay the nanny more, so why in this situation should they pay the nanny 2/3rds of the cost because they are in a nanny share? The parents with one child are not suddenly getting significantly less care for their child.

It really is not the only fair way to do it. It is ONE option, that's all.

QED - nanny shares can be very difficult to set up due to a number of factors, but assuming all else is equal (which it very rarely is) then sometimes the parents with more children will pay a bit more (maybe 100G a week), but not always and to be honest, they are very rarely 'all else is equal' so maybe one has 2 children, but is home an hour earlier, or one of the older children goes to nursery 3 mornings a week but the other child doesn't. There really isn't a 'usually' and it's all down to negotiation.

Some nannies will ask for a pay rise when another child is added to the family - some wont. Some of those that ask for it will get it and some wont. Some parents will offer it, some wont. There are arguements for & against. The majority of MN nannies tend to say they don't ask/don't get more for another child, that doesn't mean it's reflective of RL.

The bottom line is, everyone needs to be happy (both sets of parents & the nanny) if anyone of those is unhappy with the arrangement it will all fall apart - usually after a fairly massive disagreement!

'If they were at a childminder she would keep paying the same rate for her child'

Yes, but they're not. They're with a nanny. If they were with a childminder the OP would have to pay double fees, but they're not with a childminder!

Of the shares I know it is worked out as above i.e. you pay for how many children are with the nanny. It's really the only fair way to do it. The other family should get a reduction in cost as they're getting a reduction in service.

Nannies are not paid per child, although when an extra child is added there is often a small rise 50p-£1 ph extra.

There's no way you should pay 2/3 & her 1/3 - that's ridiculous. Is the nanny being paid any more? If the nanny is asking for more money (which would be reasonable) then you should pay that. If the nanny isn't asking for more then probably the nice/polite/reasonable thing for you to do is find something that would help the other parents out in some way that you are happy to do - either host the share more often or less often (which ever they'd prefer) or maybe pay the nanny to take the kids to something the older two would enjoy that the baby wont hinder.

At the end of the day, you need to come up with something you are all happy with (ALL being both sets of parents & the nanny) or the share will fall apart, but 'fair' is not 1/3 & 2/3 split. No way.

How do nanny shares usually work when there are different amounts of children in each family from the beginning rather than part way through as here? It might seem fair enough for the OP to pay more due to having more children but it seems rather unfair for the other share partner to get a bigger reduction. If they were at a childminder she would keep paying the same rate for her child.

If the nanny increases her charge (does that generally happen when a baby is added?) then it does seem right for the OP to pay the extra cost.

Nannies and childminders are paid differently. Nannies will often cost more than a childminder, particularly for one child. If the other mum is paying more for her son to be with a nanny she should get a nanny service, not a chilldminder service where extra children are taken on and you continue to pay the same.

The OP will still benefit as if her child was with a childminder it would be double fees for two, here should would only be paying 16.6% more than she is now (difference between 50% and 66.6%).

I think your friend has a fair point, but maybe you can use the change in cost share to get some additional benefits for your money?

E.g. If it is more convenient for you when the kids are looked after at your house, because you don't have to worry about drop offs and pick ups, maybe you could negotiate 2 days at yours, then 1 day at hers rather than an equal rotation of days.

I think the nanny probably does a lot of things with the children together, not say putting 1 child on a seat for an hour and doing something with the other. Take childminders for example, other peoples fee doesnt go down just because they get another mindee. The nanny should be the 1 who benefits out of the extra child (she is doing the work) rather than the other parent.

I think she's right too.
You should be paying 66.6% and she should be paying 33.3%

I think she's probably right tbh. At the moment your DD get's 50% of the nanny's time and her DS gets 50% of her time; you pay 50%, she pays 50%. If your DS joins the share her DS is now only getting 33% of the nanny's time (your DD gets 33% and your DS gets 33%). It's fair then that she pays 33% and you pay 66%.

If the nanny isnt being paid anymore then the share should stay the same IMO. If however the nanny is charging more then you should be responsible for the extra cost.