The reason why the appearance of daughters and sons in the character 好 means good or beautiful can be explained with the aid of the Chinese culture. Other websites describe it better but basically, in Chinese culture (generically speaking), parents find their children unconditionally good and beautiful and think that their children are kindhearted the minute they are born. Also, another explanation implies that a family is good because they have sons and daughters, because the children embody the continued existence and happiness of a family. Since time immemorial, the Chinese put much emphasis on the value of a family and kin, which is reflected in traditional practices of ancestor worship.
I'm sure these values are shared in other cultures around the world. Nonetheless, this provides an explanation for the link between having a son and daughter, and the why my life is good and beautiful, 好.
Thank you for letting me share this part of my world with you!