What you could do is save the household and then manage the world, there are settings in there, you can check your households and others there, you can select one of those households and then head into CAS for the household, you can make approximations between the child of the parents and give them 50% of their mothers and fathers appearance as the game itself picks one of the family members and replicates the exact look on the child.
So if two parents were raven black and blonde hair colours; one had brown eyes and the other had green eyes; one also had a deep tan and the other was pale while: you can approximate auburn hair and brown-green eyes with a fair skin colour from household CAS when making changes. You can adjust a few other features, but you should wait until the child becomes a teenager to make any signifcant changes to the face and body shape as toddler and child options are limited. If you do it right, each generation can look different rather than trying to make minor changes from the dresser. I hope that helps.
