The NCMA advises childminders to negotiate payment for bank holidays that occur on a contracted day. So if the holiday is on Tue only those parents who have booked tuesdays would be charged.
The NCMA advises that if a charge is not made on the bank holiday, the hourly or weekly rate should be increased to cover the loss of earnings through bank holidays in the year.
A few people have posted that their nursery does not charge bank holidays, if you look through older threads its clear that some nurseries do charge when they are closed. Similarly, not all childminders charge full fees.
So you need to look at the overall cost for the year, comparing hourly/weekly fees and the quality of service you will receive.
Its not easy to make a living out of childminding. We have huge expenses and we don't have the flexibility to accept every enquiry we receive because of the restrictions imposed by Ofsted & our public liability insurance. Also, once you take on a part timer, its hard to find another family requiring the remaining hours. A plumber, for example has more flexibility to fit jobs around each other each week as people are willing to take time off work for short term jobs.