Depending on the abuse your husband suffered from his parents, I think he has already put your children at risk. He may have in the past been on the watch for any strange behaviour from both his parents and the children BUT you were not privvy to that information at the time.
Unfortunately, I think your husband has to get his act together and come up with a plan that suits you as the mother of his children. I agree with karin4him the protection of your children is your no.1 priority, even if it means you disturb this "happy little family". In the mean time I would not leave them alone with his parents for a second.
However, hang in there for your husband, it is early days and he will come around eventually, with your support he will start to heal but it may have to come to him having to severe ties with his parents.
South African Male Survivors Of Sexual Abuse
Web page www.samsosa.org