Play with Genetics makes a child look like the parents. Example from my family: There is is strawberry blonde woman with blue eyes and a brownish red guy with purple eyes. If I use PWG, the possibility can be a blue eyed brownish reddish child or a purple eyed strawberry blonde.
About changing traits, there's a lifetime reward and a cheat to do that. (Cheat-testingcheatsenabled true, shift click on sims, modify traits.)