I put this in my handbook,
UNDER 5?S IN OTHER SETTINGS
A child is counted as an under 5 by Ofsted until they are in full time education i.e. at school for 10 sessions a week.
A child who is at pre school is a under 5 and if they are with a childminder before and after pre school, and require a fulltime space during the holidays they therefore are taking a childminders full time space. As such you are required to pay retainer fee for the time that they are at preschool, as the childminder cannot then fill the part time that they are at pre school.
but it really is up to the childminder how she runs her business, ask her.