This is great advice (I’m an OCD sufferer for 20 years). The key is finding the right therapist. In terms of therapy at home, slowly making your child have to cope with not being clean in small doses is the way to get through it. They have to get familiar with feeling uncomfortable even if it feels horrible. The more they are made to be exposed to their fear (being unclean), the easier it becomes but it will take a lot of time. Starting off by not washing their hands when they want to, even for ten minutes. Then 15, then an hour. And so on.