What is the ruling on doubting the impurity of a car seat due to a child's urine, or its impurity from the humidity of rain or cold liquids?

The questioner is afflicted with purity obsessive-compulsive disorder (waswasat al-tahara). The cure for this is to know that the default state of things is purity, and a pure place is not judged to be impure except with certainty, not with doubt. So, if she doubts the impurity of a car seat, or something the child holds, sits on, or walks on, the default state for all of that is purity.