Sometimes my child's impure clothes touch my clothes without any wetness appearing on them. Do they need to be washed for purification? Are there other methods of purification, such as exposure to the sun? Is there a specific age at which a child is no longer considered impure?

All praise is due to Allah.

A child's urine is always impure, but during infancy, before the child begins to eat food, its impurity is mitigated, and sprinkling and douching suffice for its purification. After the child starts eating food, washing becomes obligatory, as stated in the Hadith (definition). If a child's garment is wet with impurity and touches a pure garment, then if the wetness is obvious, the pure garment becomes impure and must be washed with water. However, if the wetness is slight, it does not have an effect. Impurities can only be purified with water; the sun alone is not sufficient.