My sims rarely have kids to begin with, but if they age up with the no chin glitch or something that is otherwise outrageous, I will edit that slightly. I wouldn't edit them to make them look pretty, just to make them less freaky. I still want to preserve the parents genes the best I can.
However, I do often make kids in cas through the genetics feature and I usually randomise until I get a decent looking kid. Which boils down to the same as I mentioned about born in game sims. I like to preserve the family genetics, but faces that are really off will be tweaked slightly.
I never edit body weight though, then I'd just give the the right kind of ice cream in game ;)
