I read the Elder and Younger Edda (basically the collection of Nordic mythology) and I know about Sleipnir but I totally don't remember any passage that'd say anything about Fenrir and the Midgard Serpent being Loki's children as well.
Although I might've been too distracted by that one story with Thor in a bridal dress to remember.
Okay technically there are a lot more than 6, since Loki's a manwhore who likes to have sex with men while in the form of a woman and gets pregnant a lot, but there are 6 "important" kiddies:
Fenrir the Great Wolf.
Slejpnir, an 8-legged horse that Odin immediately claimed and named his official steed.
Jorgamund The Midgard Serpent
Hel, guardian of the Norse Underworld (not the same as Valhalla)
Then there's Narfe and Vale, whom he had with his actual wife Sigyn. At one point one turns into a wolf and accidentally kills the other.
There's also a troll girl that I believe he birthed while in the form of a human man.
How do you screw a snake and a horse?
So he can run, like really fast, duh.
Although I might've been too distracted by that one story with Thor in a bridal dress to remember.
Fenrir the Great Wolf.
Slejpnir, an 8-legged horse that Odin immediately claimed and named his official steed.
Jorgamund The Midgard Serpent
Hel, guardian of the Norse Underworld (not the same as Valhalla)
Then there's Narfe and Vale, whom he had with his actual wife Sigyn. At one point one turns into a wolf and accidentally kills the other.
There's also a troll girl that I believe he birthed while in the form of a human man.