Unfortunately, if you want alien babies then your MALE sim has to become pregnant from an alien abduction or TWO aliens need to make the child.
if you're looking for a hybrid IE Alien Male + Human Female or Alien Female + Human Male you'll get a hybrid of sorts that doesn't have access to all of the aliens powers but will have the skin and such form the alien.
and Vampire Alien hybrids where the children have the chance to and can access full alien powers are not possible apparently.
but you can always cheat your way to having alien children.
http://sims-online.com/aliens-sims-4-get-to-work-male-pregnancy/
http://sims.wikia.com/wiki/Hybrid
https://forums.thesims.com/en_US/discussion/910592/do-you-think-alien-vampires-can-be-a-thing
My Vampire sim just gave birth to a vampire alien hybrid
