I charge per day, and the same for all kids regardless of age. If they don't come, I charge anyway. If I am closed, they are not charged for that day.

I would up it to $33. You can get up to $35 by next year. This isn't a lot for most parents, and if they like your services (which they clearly do, as they're all still there), then $15 a wk shouldn't send them running.