I think you should definately write to ofsted and inform them along with any paperwork you got from the police which states that they are taking no further action, I would also tell them what steps you have put in place to monitor the situation.
I feel very at your situation, and can understand that you feel some of your parents might take it badly.
I got a link on here once about outcomes of ofsted complaints (sorry but can't find it now I think it was from a post nannynick did, if he reads this perhaps he will be kind enough to pop it back on) anyway it made quite interesting reading, one of the cases was about a minder who's son was caught with drugs and minder hadn't informed ofsted, her son's prob was obviously a lot more severe, they found stuff on premises and their were some other issues as well, but the key info was that the minder did not inform them of a significant event relating to a person living on the mindees premises (someone contacted them anon) the minder lost her reg, perhaps if she had contacted them first she may not have.
Please don't loose heart and give up minding, the fact you are so upset proves that you care and want to do the right thing with as little damage, I'm sure your not the first minder that has had this problem and you certainly won't be the last, I'm sure ofsted will be very helpful I would suggest you speak to someone in compliance, rather than switchboard staff as they should be able to advise you straight away, I doubt very much that they will insist you tell the parents of mindees, as I understand it disclosure is only if they give action (make sure you send in risk assessment etc).
Best wishes whatever you decide and good luck