Does a clean bed become impure if a child sleeps on it and sweats at night, and the original (unwashed) bed was previously affected by urine?

The child's body does not necessarily become impure (najis) if he sleeps on a pure sheet while the mattress beneath is impure, as the pure sheet might act as a barrier between his body and the impure mattress. However, it may become impure if there is no such barrier. Similarly, the pure sheet becomes impure if the child's impure body sweats on it.