They won’t register as being half-alien, I don’t think. The game only seems to recognise full alien or non-alien. If they’re full alien, they will have been born with a different outfit to human sim babies, plus if they were born in the hospital, you should have ended up with the alien crib. In my experience, if a sim has an alien baby through woohoo rather than abduction, the babies won’t register as being alien and will only have a sim form, not an alien one as well. Best way to tell whether that is the case is when they age into a toddler and you go into CAS.