"Alle lakens zijn gewassen door mijn zoon."
Translation:All the sheets have been washed by my son.
"Are washed" would imply some recurring event: in general, my son washes the sheets. In the Dutch passive construction, you need to use the auxiliary verb "worden" (i.e. present simple) for that.
On the other hand, "zijn gewassen" describes something that happened in the past and is now done (present perfect). The sheets are now clean, the son washed them. They were/have been washed by my son.