I think it be hard to charge parent for stat if it not their scheduled day. If they working 3 day a week and budget for 3 x daily rate, it mean you going charge them 4 days if stat in week when it not their day? If Christmas Day is on Saturday or Sunday, then do you charge 6 days to your FT client?

I would do normal contact but state which 3 days they always be in and what day the 4th day is when that happen. Then if you on leave on their registered day or if it stat day, you get same pay you normally have but have not worked the stat or sick/personal day booked.