How can one deal with the obsessive thoughts of clothes and belongings being contaminated with a child's blood, especially after learning that blood is impure and storing winter clothes without washing them, given that the patient already suffers from obsessive-compulsive disorder?

It seems the questioner is still suffering from waswas (obsessive whispering). It is recommended to continue treatment, disregard the waswas, and pay no attention to it. Blood is impure, but a small amount of it is excused on the body and clothing. So, if a small amount of blood gets on children's clothes or toys, there is no harm in that. When in doubt about something being affected by impurity, the original state is to presume purity until there is absolute certainty of contamination.