Reducing hours and childcare payments: ask the expert
This sounds like a contractual issue and possibly a mis-communication. Either way, you should check the contract or agreement you have with your childminder for notice requirements for changes to the hours she looks after your child and try and talk to her about this. If you gave your childminder the necessary notice in writing of your change in hours, as stated in your contract, then she should only be charging you for the hours that you are now using. As childminders are self-employed and they need to fill their places, which can take time, they may require more than one month's notice of a reduction in hours. This should have been explained when you started using the childminder and when you gave notice of the reduction in hours.
If you let your childminder know about the reduction in hours verbally rather than in writing, it may be slightly harder to prove that you have given the necessary notice so it's important to speak to her about this. You should give her the date and details of your conversation of when you gave notice and explain that you thought that this was adequate. If there has been a mis-understanding hopefully you can resolve it by talking it through and negotiating with your childminder.
If you feel that you have done everything correctly and have given the required notice, you could speak to the National Childminding Association for advice on breaches of contract. Contact NCMA: 0845 880 0044. Alternatively, you could contact your local childminding network co-ordinator who should be able to advise; your local Family Information Service will be able to give you their contact details.