The CAB guys sounds rubbish. Ignore him.
You had a contract with the childminder and you owe her money under it if she meets her contractual obligations the MOST important of which was to keep your child physically safe. She failed to do that so she, not you, first breached the contract. You ‘accepted her breach’ and the contract came to an end.
(Also, she can only get money out of you for a breach of contract claim if she takes you to court, in which case there will be public evidence on both sides including what her son is said to have done. Seems unlikely she will do that, especially as she would probably lose.)
I wouldn’t pay her a penny. I would send her a formal letter saying something like:
Dear x
I write further to your message of requesting that I pay you fees for the notice period of our contract. I was very surprised to receive your message.
Our childcare contract did not come to an end because of any notice from me. It came to an end because you failed in your legal duty to keep my child physically safe, which was a breach of contract by you, as well as a huge safeguarding issue which as you are aware has been reported to the police and other authorities.
Also, when I raised the matter with you, you said that you would not be providing any more childcare to me, a further breach of the contract.
The contract thus came to an end as a result of the breaches by you, and no further sums are due to you from me under the contract. Should you feel otherwise, you are of course free to commence legal proceedings in a court. Please note that should you choose to do that, I would bring a counterclaim for my lost earnings caused by your breaches of contract.
All rights are reserved, including the right for me to take legal proceedings against you without further notice to you, which may be for my lost earnings as well as your culpability for the assault on my daughter and negligence in that regard.
Regards,
OP.