I think most providers would give at least 2 weeks notice of a change in operational hours. I think at minimum I would give 1 month, and likely 3 months notice of the change. For me I would say it voids the current contract, as the agreement has changed. So I would issue new contracts for the parents to look over, and decide if the changes still meet their care needs.

Did you have any prior issues with the provider? Any late payments, or late pickups? Any behaviour or developmental concerns with your child? You mentioned you may have been the only family impacted by the hour changes. It could be as simple as the provider felt a need to shorten her working day. Sometimes though a provider will shorten their hours to nudge out a family they are not comfortable terminating care with.

You may be able to approach her on the fact that she voided the current contract by changing her hours. Someone else may be able to better advise on that. I'm sorry it didn't work out, and that the changes left you scrambling for care. I wish you all the best at your new daycare.