When I first registered and didn't have a lot of work, I hoped that all the schoolchildren would come full-time in the holidays, and that I wouldn't have any parents needing childcare who worked in education!
However, I now have several schoolage children before/after school who do not require full-time care in the holidays, and I have also minded a student's baby and a teacher's baby!
I'm very laid back about it now. If parents want term-time only, then that's what I will do, whatever age the children are.
My schoolage childrens' contracts are very specific about days/hours during term time. The contracts state that if they want childcare during the school holidays, you basically pay for what the child does. It means that, sometimes, I have an absolute houseful of children (all within my legal numbers!) - at other times, I may have no schoolchildren at all. My situation is lucky in that I have a lot of children on my books, and it all seems to balance out well and I've never yet had to say "no" to a parent because I've already got my quota!
Parents know that I prefer them to decide before the school holidays what they would like, but I still sometimes get a frantic phone call on a Sunday afternoon asking if I can have their child the next day, and I'm really OK with that. I think that I am unusual in that respect, as most of my minding friends want to know well in advance what they will be needed for, and many would charge a retainer if not required.