I am exactly like Playfelt. I freeze rates for current clients. Their rates last no more then 3 years since then the child will be in school. Then they are set to the current b/a school rates.
As soon as a client goes on mat leave, their rates go to the current mat leave rates and if they come back full time they go up to the current daycare rates at that time.
All of my clients know this and appreciate that I'm not raising their rates unless their situation changes. It's a bit of a pain from an accounting and tracking point of view, but it's what works best in the long run for the daycare.