I'm not following the logic of the '50:50' argument at all on this one, and as OhFraktiousTree has expressed the issues in more depth (as I would expect
) I am going to follow her lead:
If there's no payrise now but the other family have another and them there is a payrise AND it goes back to 50/50 but on the new figure how does that work?
Both families would have two children cared for so they split the cost 50:50 - where is the problem?
I don't think it's fair in the OP for the other family to get cheaper childcare
The corollary of that is that you do think it is fair for the family to get twice as much childcare for the same price! Why?
equally I don't think it's fair on the other family to get proportionally less attention
Now that I can agree with 
One could split the costing out according to the relative convenience of a nanny (equal) which is part of what attracts a premium
Yes it is part of what attracts a premium, but equally there is a premium because my child(ren) are getting the whole of a nanny's attention (or in the case of an equal share, 50% of a nanny's attention) as opposed to say 25% with a childminder with 3 other charges. As soon as the share becomes 2+1 the other family is not getting that 50%, they are getting at best 1/3, but as a 9 month old requires a lot of looking after it's probably less than that!
Plus as a nanny I wouldn't have been happy knowing that the costing had been altered so drastically because then I'd feel that I wasn't expected to provide an equal service to both families.
Absolutely you would be expected to provide more service to the family that is paying more - how else would you cope with the fact that they have twice as many children, one of them only 9 months? Spend 50% of the time with the only child, 50% of the time with the 9 month old and ignore the older sibling???
Would they expect to shoulder more of the cost if the OP hadn't had another baby and instead sent the older one to preschool?
No because this would not be their decision: they may get more of the nanny's time for their money, but they didn't ask for it so can't be expected to pay for it.
Or will they expect a reduction if they decide to do 2 mornings preschool?
No because they would be deciding to change the arrangements unilaterally and cannot expect the other family to bear the consequences.
Also don't forget that the other family is going to have to put up with a whole load more equipment at their house and the additional wear and tear of a child that isn't theirs. I think 2/3 + 1/3 is a bargain!