Do only set days, and word it very specifically what the days are in the contract. Don't do switching or rotating schedules week to week. Make the part-time rate higher(mine is $5 day higher depending on age group) They pay for regular scheduled days every week regardless if they attend or not. Remember to include stat holidays(if in your contract) if they normally attend that day. I do a 3 day a week minimum attendance. Extra days require extra payment, even if they didn't attend one of their normal days that week.