So looking at the picture and with the images of known dragons from Heavensward now, I think we can extrapolate the following:
Red (Centre) is clearly Midgardsormr
Orange (top left) is Hraesvelgr
Purple (second bottom) is Nidhogg
Pink (bottom) is Tiamat (the shape of the horns is distinctive)
Based on the Norse mythology the name of Ratatoskr has something to do with a tusk, and some pictures of the squirrel give a unicorn horn, so it is possible that the Dark Blue (top right) “Narwhal” dragon is Ratatoskr
As for Bahamut, I think he might be the Green?? If you consider the horns (which isthe major identifying factor in these images), and ignore the gold Allagan junk, Bahamut has backward sloping horns like the Yellow one, but he neither has the green colouring or the spiky neck-frill things or a long serpentine neck. The Light Blue one has the kind of stocky build and shorter neck as well as plain leathery wings, but the horns are completely different coming around the front of the face. And the Green one has got the backward horns, and shorter neck, but he looks a lot more feathery. That said, there is nothing to say that the image of Bahamut given us as the primal has anything to do with what he looked like as a living dragon.
So I think that the Yellow one is one of the unknown children and the Light or Dark Blue is the other unknown dragon.
And to summarise the known information of the First Brood:
Name: Midgardsormr
Gender: Male
Status: Alive
Notes: Father of the First Brood. Boss of Keeper of the Lake. Original body still wrapped around the Agrius. Form reborn first as a minion, then as a mount by aether drawn from Hydaelyn.
Attitude to Mankind: Benevolent, companion of WoL
Name: Hraesvelgr
Gender: Male
Status: Alive
Notes: Fell in love with an Elezen woman, Shiva, who he consumed (voluntarily) in order to bind them together forever and forge peace between people. Gave an eye to his brother Nidhogg, but had it returned by Wol/Estinien. Last seen delivering Ysyale to Azys La
Attitude to Mankind: Indifference? Not “fond” of humans, but any anger is tempered by Shiva’s influence.
Name: Nidhogg
Gender: Male
Status: Alive
Notes: Started a war with the Ishgardians in revenge for killing Ratatoskr. Lost both eyes to Prince Haldrath which were then used by the Ishgardians for different purposes. Boss of the Aery and killed by WoL/Estinien. Form reborn using his eyes and Estinien as a vessel.
Attitude to Mankind: Aggressive
Name: Ratatoskr
Gender: Female
Status: Deceased
Notes: Killed by King Thorodan & crew for her eyes
Attitude to Mankind: Unknown
Name: Bahamut
Gender: Male
Status: Deceased
Notes: Mate of Tiamat. Killed by Allagans while defending the dragon lands from being invaded.
Primal Version: The primal Bahamut was an imitation of the real thing summoned from Tiamat’s sorrow with help from Ascians. It was then captured and harnessed by Allagans with help from the Ascians before being sealed away and launched into the atmosphere to become the lesser moon Dalamud. Was returned to Eorzea by Nael van Darnus and defeated by WoL (stopping his physical regeneration and sending the primal force back into the void) inside the Binding Coils.
Attitude to Mankind: Original – no idea. Primal – Aggressive
Name: Tiamat
Gender: Female
Status: Alive
Notes:Mate of Bahamut. Currently bound by Allagan technology inside Azys La. Believes her entrapment is atonement for letting her emotions overcome reason to unleash the destructiveness of Primal Bahamut into the world. Only wishes to be left alone in her penitence.
Attitude to Mankind: Indifferent??. Angry at Allagans for what they did, but not really Aggressive.
Name: Unknown
Gender: Unknown
Status: Unknown
Attitude to Mankind: Unknown
Name: Unknown
Gender: Unknown
Status: Unknown
Attitude to Mankind: Unknown
Possible remaining Offspring
Based on Final Fantasy tradition: Shinryu
Based on Norse Mythology: Veðrfölnir
Other Named Dragons:
Tioman – Mate of Nidhogg, end boss of Sohm Al
Vishap – Enormous Dragon, boss of Steps of Faith. One of Nidhogg’s horde, possibly a General.
Twintania – Dragon bound inside Dalamud and found inside as boss of Binding Coil Turn #5.