(Oops, they are aliens.) Aliens not having alien babies

I have never had a pure blood alien with a pure blood alien not having an alien child. I have seen many a non-alien sim with the alien skintones. I would first verify that both of the sims are actual aliens, by making sure they can disguise self.

Babies do not have disguises. They appear in the crib with their alien skintone, toddlers can then put on a disguise when aged up.