Actually, if you noticed, their family name has been based on Norse Mythology thus far.

Midgardsormr
Nidhogg (male)
Ratatoskr (female)
Hraesvelgr (male)

Only 3 of his children have been confirmed. I doubt Bahamut and Tiamat are part of his family.
Though I could be wrong and have missed out something part of the story somewhere..