Caregiver needs twelve days off in September for her daughter's wedding

She asked well in advance, which I appreciate, and of course she should go. The problem is mine to solve. I have two site visits a week that can't move, and the relative who covers the odd day can't manage twelve in a row. An agency has offered a temporary caregiver. The part I can't predict is how a baby who has spent every weekday with one person since he was a few months old will take to a stranger for two weeks. For anyone who has covered a long caregiver absence, how did the days run? Did you have the new person overlap with your regular one for a few days first, and was it worth paying both for that week?