Why is Chi-Chi less of a a nag on Goten?
My take is basically the same as those commercials that start with the worried obsessive mother and the first child:  She is so frantic about everything she ends up messing things up.

Second child happens, and she has everything nailed down.  Experience from the first child has her less stressed with the second child, and she knows a bit of what to expect and how to handle things.