Are the parents held accountable for the sin of their son if he uses the mobile phone they bought for him to watch forbidden things?

If the parent gave his child a phone for permissible use, and the child used it for something forbidden, then there is no sin on the parent, because it is indirect and unintended assistance. However, if the parent knows that his son is using the phone for something forbidden, then he must prevent him from using it for that forbidden thing; either by taking the phone away from him completely, or by restricting it to ensure that he does not use it for the forbidden thing, due to the saying of Allah Almighty: {O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones}.