I created a set of Default Replacement baby skins for Sims 4 which includes pale,medium and dark skin tones for both boys and girls.I created the hairstyle of boys different than girls.This set doesn't include alien baby skins.The download link and install instructions are given at the end of this post.

Notes:

• These are default replacement baby skins which means you can install only one baby skin file for one type of skin tone at a time.For example,you can install only one Pale skin tone mod for baby girls at a time,if you need to install another Pale skin tone mod,you must remove the previous Pale skin mod file from your Mod folder.

• Once you install this set,these skins will automatically replace the skins of all the existing babies in your game.Also,new babies will be born with these skins and their skin tones will be determined by genetics of their parents.

• Deleting the mod files of these skins won't corrupt your saved game having babies,they'll simply go back to their EA baby skins.

• Babies with these skins won't have any issues while aging up to child.I've tested it in my game.

I just couldn't manage a baby boy with dark skin in my game,so below is the Sims 4 Studio preview of my dark skin for boys.Thanks to Orangemittens  for providing the testing texture package file for babies.