I take this question as asking whether part time children pay for vacation (theirs and caregivers) and whether they pay for periods where no care if required.
You'll find MANY caregivers no longer do part time and if they do it is one child paying full time fees.
It is up to you. It is fully reasonable to require a family to pay for care each and every week of the year that you are providing care. Doesn't matter if grandma comes to town and wants to keep the kids home...you still need an income. Some caregivers are more casual and just charge for time care is provided...which is perfectly fine if that is all the caregiver wants/needs. The vast majority of caregivers NEED a consistent income year round and this is reflected by less part time spaces available in reliable, long standing home daycares.