Are there any sources that discuss a parents obligation to respect their (adult) children?
In Shulchan Aruch there is discussion about children respecting and honoring their parents. There is no specific mitzvah for a parent to honor his child; however we are commanded to respect all people. Parents should not treat thier children in a condescending fashion. The Chida in his commentary to sefer Chassidim 343, (see The Fifth Commandment pg 111) does say that a parent should treat his older children like a brother.