The majority of their expenses will be salaries and rent, not nappies and food. A bank holiday day will cost them nearly as much as an average day.
It probably is more sensible to spread the cost across all the days though. Let's do the maths...
Nursery open 52 weeks per year, 5 days per week except for 8 bank holidays, 3 of which are fixed on Mondays, one fixed on a Friday.
Let's say the other 4 fall one each on a tue/wed/thu/fri, though happy for others to try different assumptions and see what happens.
Nursery needs to make £50 per day per child every day ie £13k per year (52x5x£50). To compensate for the 8 bank holidays, fees go up to £51.59 a day (£13k/(260-8))
So someone using the nursery tue-thu now pays 52x3x51.59 a year, less 3x51.59 for the three bank holidays on their days. That's £7,893.27 per year.
And someone using it mon-wed now pays 52x3x51.59 less 5x51.59 - that's £7790.09
Before the mon-wed and the tue- thu person were both paying 52x3x50 ie £7,800.
So hurrah for the £10 saved!
If my sums are wrong, mea culpa...