Once a child marries they are no longer qualified to immigrate as a "child" and
are immediately categorized as married sons or daughters.
U.S. citizens can
sponsor unmarried and married sons and daughters, but the waiting period can be
quite long based on visa availability.
There is no visa category for the married child of a permanent resident.
A petition for an unmarried child of a lawful permanent resident will normally
be automatically revoked if he or she gets married. If you think a child may
marry before they can immigrate, you may want to not only consider filing a
separate petition for them now, but also applying to become a U.S. citizen if
you are eligible.
While there is no visa category for the married child of a permanent resident,
there is one for the married child of a U.S. citizen. Filing a separate petition
now preserves that option so if you do become a U.S. citizen before the
child marries, then the separate petition could continue to be processed in the
visa category for the married son/daughter of a U.S. citizen.