For children when they age up, the only thing I will change is their hairstyle and clothing if I don't like what the game has generated for them. Now, when they age up to teen, I do the same, plus make-up and eyebrow shape sometimes, and I will change their physical appearance if they have the 'no chin' syndrome which 9/10 times they do. That's the only thing I'll consistently change on my Sims' offspring.
However, I did change a child's hair and eye color once because she aged up without either parents colors. I have no idea why, but it bugged me. It wasn't a throwback situation, for instance, a grandparents, great-grandparents, etc. colorings, in which case I would've been fine with it. I created both her parents.
