Baby penis can be washed during bath time with water and baby soap.
There is no need to wash beneath the foreskin until it is more easily retractable.
As the child matures, he should be taught to retract the foreskin gently while bathing in order to clear any collection of smegma which is a natural buildup of oils and old skin cells. Usually boys discover and learn on his own to pull back his prepuce and rinse underneath with water.
Newborns: foreskin is rarely able to be retracted
By age 3: the foreskin can be retracted on 80 to 90%
Parents or the child caregiver must not retract a foreskin forcefully, do not pull back by force, it may produce pain, tearing and even bleeding and the resultant irritation and edema may cause further adhesions.