Hi,
This is all very confusing.
I'm in Ireland, with kids going to after-school and pre-school in the same creche.
If you're children are in pre-school morning's only, is it because they are in the free pre-school year? If so, then that finished last Wednesday when all children in primary school got their Easter holidays. There will not be pre-school again until April 4th, when primary schools go back to school. The ECCE pre-school year follows the primary school calendar.
If your children attend the creche 2 days, afternoons only, then you are only allowed bring the kids there those days, so as to preserve staff to child ratios. The rate you pay is usually based on the idea of there being 50 weeks in the year, which allows for 10 bank holidays per year (New Years Day, St Patrick's Day, Good Friday, Easter Monday, May BH, June BH, August BH, October BH, Christmas Day and St Stephen's Day). The rate you are charged allows for bank holidays, so if a bank holiday falls on a creche day, you still pay for that day.
My understanding is that when kids are in the pre-school class, the staff to child ratio for the age group is 10:1, as it is sessional care. However, when you went on Friday, as it was during 'school holidays' that counts as full/part time care, and the ratio is 8:1, and the space requirement per child is greater than during sessional care. When you went on Friday, they may simply have been unable to legally accept your children as it would have thrown their ratios out.
I do think it's odd though that dress-up day was Friday, as in nearly every creche and school around here, Wednesday was 'green day' and kids were in all sorts of costumes and face paint.
My older children used to go to creche 3 days per week. If dress up day/party fell on a day they weren't there, there was no question of them being allowed attend. Ratios were to a limit, and the creche were minding the kids they were being paid to mind.