What is the ruling on places where a child sits in impure clothes, with no trace of impurity (smell, color, wetness) in those places, and is this considered doubt or certainty regarding the impurity?

Giving in to doubts about impurity opens a great door to evil. Therefore, a thing is not judged to be impure merely based on doubt, and the default is purity. Places should not be washed unless there is certain conviction of their impurity. If you are certain that the child sat down with wet and impure clothes, and you wash the place, it is more cautious and absolves your responsibility, provided you do not wash what you doubt to be impure.