1. Tony is divorced from his kids' mother (who is not Pepper, btw) and the kids are a girl and a boy. (The girl is slightly older by a year or two.) Tony has been very careful to keep his kids away from the press as much as possible and he has them for two weeks? months? every summer.
One day, they are kidnapped by Doom? and Tony freaks out. All of the Avengers are searching for them. Loki happens to show up at Doom's castle?, sees the kids and discovers they were kidnapped -- and the kidnappers meet their end. Loki slowly wins their trust and takes them away to have ice cream.
Tony and Avengers arrive a little later; when he finds out they are with Loki he panics again, even when Thor tells him that they couldn't have a safer babysitter than Loki.
Jarvis? tells Tony the kids are back at the Tower w/Loki, and when Tony gets back, he sees they are sitting around the pool and having ice cream. Loki disappears; the kids are safe and everything is ok.
2. Tony accidentally 'creates' a love spell in his lab and it accidentally 'snares' Loki, who is visiting Thor. Loki realizes it is a spell right away and blames Thor but then he realizes that it couldn't have been Thor's doing. They discover that Tony is to blame but Loki is unable to reverse the spell because Tony didn't know what he was dealing with (magic.) The spell also bound Tony to Loki; Tony can find Loki no matter where he goes.
3. This is a Thor-based AU, just before Loki is caught by the soldier on Jotunheim and sees his arm turn blue. Laufey had spotted Loki moments earlier and had Thrym and his men follow Loki to catch him. Loki is grabbed, turns blue and tries to run, only to be knocked out when one of Thrym's men hits him in the head. Loki is discovered to be the "lost prince" and Helblindi, who had been groomed as heir apparent, is now the spare. Byleistir, the youngest, is not as bright as some of the others but he is very loyal. Loki thinks he has been kidnapped and no-one tells him that Odin tried to ransom him but Laufey turned him down. Loki slowly learns to accept his heritage and eventually becomes used to it.
I know the third one is on AO3.