Baby powder is not needed or necessary with disposable diapers as long as you change your baby or toddlers diaper often enough.
With cloth diapers it's a good idea to use baby powder though to help absorb the pee from the skin of the baby or toddler in diapers.
But with modern disposable diapers they are so absorbent that the pee is held away from the skin so the child's skin should stay mostly dry unless the diaper is full of pee.
But it's okay if you want to use baby powder on your toddler or baby with disposable diapers as it can help prevent chafing from the diapers as well.
When applying the baby powder to your baby or toddler apply it to their diaper area and then put the diaper on them.
Don't put the baby powder directly into the diaper as it can prevent the diaper from absorbing the pee like it should.
Apply the baby powder to your hands and rub it on the child's or baby's skin or sprinkle the baby powder directly onto the baby's skin.
Then pull the diaper up over them.