Mr. Anchovy, let's look at it the other way.
OK, I'll try and follow your logic...
Joanne's childminder listens to Joanne's claim that she is being unfairly charged and decides thar she won't charge for bank holidays any more.
However, she still has her bills to pay, so she does what other self-employed professions do and raises her hourly fee to factor in the bank holidays.
Understood. There are 8 Bank Holidays in the UK, and 260 week days (365/7*5, normally rounded to 260). So all things being equal, a rise of 3.2% should cover Bank Holidays. If two weeks summer holiday is being charged for as well the rise is 7.4%, which seems a bit steep until you factor in the fact that you only pay that for 50 weeks out of 52, which brings the rise over a year back down to 3.3% over the course of a year.
Now Harriet is being charged unfairly.
No, she is being charged more per day, but she was being charged too little before (and Joanne was being charged for too many days). Now everyone is paying the same rate per day for the number of days the childminder is available to care for their child.
Although she never gets the bank holidays,
You mean she never gets her child cared for on a bank holiday? Well nor does Joanne.
she is paying towards Joanne's and her childminder's bank holidays
Not sure what you mean by paying towards Joanne's bank holiday. Yes you could say that she is paying towards her childminder's bank holidays now: before she wasn't, Joanne was paying for them all herself which is why it wasn't fair.
and - get this - as an employed person, Joanne is being paid for them anyway!
I am not sure that assumptions about Joanne's employment status are helpful - how about assuming that Joanne needs a childminder every Monday because she has to care for her disabled sister that day? Alternatively, assume Harriet is also employed and gets paid for bank holidays.
And what about poor Laura? She sends her 2 kids on Tuesday, Wednesday and Thursday, never gets those bank holidays off, yet pays 6 times as much towards Joanne's and her childminder's bank holidays. How fair is that?
None of Joanne, Harriet and Laura get any care on Bank Holidays so they are all equal there. Under the new arrangements, nobody pays any money for the care that is not available either, so that is also fair. Everybody pays the same rate per day of care, which is 3.3% more than Harriet and Laura used to, which is also fair. Before, Joanne paid over 11% more than Harriet and Laura per day of care. Which was not fair.
Whether or not Joanne or Harriet go on holiday is their own affair and beside the point.
The issue of charging for the childminder's holiday is more subtle in its unfairness, but it is not beside the point. The point is that the childminder is unfairly charging everybody for something they can't have (childcare during the two weeks of the childminder's holiday). My example illustrates the unfairness of this because it has a greater effect on Joanne than it does on Harriet, but it is only an example. Perhaps you would prefer the example of Nina who leaves at the end of summer term and Penny who replaces her, starting at the beginning of the summer holiday. By the end of the holiday, Penny has paid for 6 weeks child care and only received 4!
Finally I should point out that nothing I have said implies that it is unfair to charge people more per hour in term time than the holidays, or reduce fees for siblings, or even charge more for Mondays because you want to put people off because you want an easy start to the week! Nor is it unfair to charge during parents' holidays, retainers to hold places open or any other charge for when the service is available. And am I not saying that it is illegal to charge for Bank Holidays, or that Childminders are not free to offer whatever terms they like (within the law).
What I am saying is that if you want to be fair and professional, only charge people for what you are prepared to provide.
sofewbraincellsleft, my apologies for hijacking your thread.
I think the only apologies due are mine: I started the argument, and I am sorry if this has caused offence to anyone including the original poster. In my defence, I did answer the original question which was how to add an effective rider to a contract.