I don't increase my fees for existing clients, and sell this as a positive benefit - a sort of loyalty bonus. Parents do compare notes, but even those on a slightly higher rate can see that they will in due course be paying less than subsequent newcomers.

I also have different rates for different numbers of hours, so those who are close to full time are paying less per hour than those who only do odd bits. If parents reduce their hours, they go onto the then current rate for the number of hours they drop to. This makes me feel better about it, if nothing else.