I am not a childminderf, but I have frinds who are.
The comment made earlier about the parents paying if THEY go on holiday is right. When all's said and done, the childminder is doing this for money after all, and they cannot just mind another child in the meantime, so they should expect to be paid.
However, to your question, unless it was agreed from the outset, I ( and one of my friends who I have just been speaking to about this) would say no, you shouldn't pay. In fact, of the childminders I know (6 or more) they arrange their own holidays to co-incide with school and nursery holidays, so they take the time off when they are least likely to be needed.