From what I've researched about supernaturals, it's something like this:
If parents are a ghost (CAS-created) and vampire, possibilities are:
- 25% for a human-ghost, 25% for a vampire-ghost, 25% for a vampire and 25% for a human.
About the "Magic in the air" moodlet, i've heard about it resulting in occult babies, even with fully normal parents. I've not myself had time to test it, but it's said in the Life Status section at
http://sims.wikia.com/wiki/List_of_MoodletsNow, off to test that moodlet!
Edit: Now I've tested it, and two humans were able to have a witch baby with that moodlet. So I think it may allow them to have other type of supernatural kids too.