I would be asking the one that has been normally taking time off if they will take wed off so you can take those needing care as in make it more of a will you rather than can you. Then if she says no then tell her fine if you can't help me and the rest of the group out this week after all of the things from the summer then there will be no free days this week. You could also approach it that way in that due to the fact there are more kids needing care on Wed then you have spaces for IF they can not take the wed off as a non pay day then there will be no other non pay days available that week. That is fair in the sense of you are still allowing them the right to come if they want to but also not rewarding them for not helping you out - both parties doing their part to work around a situation.