All parents are a little disappointed in their children. They're disappointed their children didn't turn out like they did.
Some cases are obvious, like when the parents are strict religious conventional types and the kid becomes a pothead atheist punk/goth kid. This happens a lot. Some cases are less obvious, like when the parents are pothead atheist punks/goths and the kid joins the Latter-Day Saints. This happens more than you think.
However, all parents are disappointed their kids didn't turn out like they did. They want their kids to be younger versions of them. They're happy when their kids act like they do and they don't like when their kids are different. They want their children to turn out the way they wanted. They may know someone who they want their kid to be like and they're disappointed their kid didn't live up to it. Whatever it is, parents are mad their kids didn't turn out like they did.