"But you were destroyed,
in the first year of the time war at the gates of Elysium.
I saw your command ship fly into the jaws of the Nightmare Child.
I tried to save you."
Faults I may have with Russell T. Davies and his handling of 'Doctor Who', but I do give him his props in various areas. First off, that he was able to revive 'Doctor Who' from its long slumber and in such a way as to make it probably more popular than it ever was.
And then there's his creative turns at coining names. The Moxx of Balhoon, the Clan Slitheen, Raxicoricofallipatorius, Klom, Adipose -# Well, that was just lazy, or maybe the TARDIS translation of their name in their native language.
And then in "The Stolen Earth", the Doctor mentioned the fate of Davros in the jaws of the "Nightmare Child". (I'm wondering if he was inspired by the KISS Psycho Circus computer game?)
No other mention is made of the Nightmare Child, but even so, the TARDIS Wiki had a splainin as to what it was:
The Nightmare Child was an area large enough for a ship to fly through the mouth of, it was near the Gates of Elysium, during the Last Great Time War. Davros' ship was damaged near the Nightmare Child and the Doctor witnessed him fall into its jaws, and attempted to save him.
Dalek Caan, who escaped from 1930 using a temporal shift flew into the jaws of the Nightmare Child, saving Davros in the process. (DW: The Stolen Earth)
[Retrieved from http://tardis.wikia.com/wiki/Nightmare_Child]
But they don't offer any official authenticity for this interpretation, or their source for making this claim. So I think whoever wrote up this splainin must have made it up from whole cloth. Which is fine, (I'm about to do the same thing!), but say so.
I'm thinking the interpretation of the Nightmare Child is wide open, and until it is finally specified within the show itself, (perhaps in a "Time War" movie starring Paul McGann? My Little Buddy Sean would be so happy!), then we can speculate on our own interpretations of what exactly was the Nightmare Child.
And of course, with me being a televisiologist who believes in most TV shows sharing the same spatial plane of existence, I have my own suggestion for the Nightmare Child:
Here's how "The Doomsday Machine" was described in the 'Star Trek' Wiki, "Memory Alpha":
The doomsday machine, also referred to as the planet killer, was an automated, self-propelled weapon capable of destroying entire planets. It was encountered in 2267 by the Federation starships USS Enterprise and USS Constellation. Its origins were unknown, but based on its apparent trajectory, it was believed to have come from outside the Milky Way Galaxy.
This planet killer could have been created by either Davros and the Daleks, or by the Gallifreyan Time Lords, with the intent of destroying the other's home planet. Or it could have been one of the lesser participants in the Great Time War, with the intentions of destroying both of those major combatants.
Somehow the doomsday machine evaded being trapped within the time-locked parameters of the Time War area (probably busted its way out!) and then traveled through space until it encountered Commodore Decker's ship the Constellation.
Like I said, this is just a theory and will be discarded once there is official verification of what the Nightmare Child actually is. But until then, it makes for a nice way to combine 'Doctor Who' with 'Star Trek'.
The Time War is another option, linking 'Doctor Who' with 'Enterprise', but that discussion would probably take a LOT of postings!