Details: The fic takes place Post-Thor: The Dark World or incorporates spoilers from that movie since Loki is on the throne and Frigga is dead. Anyway, Tony and Clint are on Asgard as ambassadors of Midgard. (Tony possibly considered royalty and Clint representing SHIELD.) Loki (as Odin) holds a feast and invites delegates from the other realms. So people connected to the royalty of the other realms come to Asgard to talk. (I.E. The royals don't attend the event themself, they sent their underlings.) There a procession where each visitor is announced to the crowd. Among those attending are a few of Frigga relatives, who have come to give their condolences. While invitations are sent to all the realm, no one expected a delegation from realm Helheim/Niflheim. However, after the other people from the various realms have been seated, a delegation from Helheim arrives. There delegation consisted of a small female child and an adult woman. The child is the Queen of the realm. They are announced and then seated. The Queen of Helheim is seated next to Tony and basically tell him not to trust Loki, but Odin instead. In other words, the Queen knew that the Loki walking around Asgard was an illusion and that Loki was disguised/impersonating Odin.
The author also had some drawings associated with the fic. The one drawing I remember is one where the author drew out each realms crest and showed where each person was actually sitting in that scene. (I.E. The drawing showed the seating arrangement of those participating in the feast.)
Edit (6/2/2015): I found the fic I was looking for; it's Dance a Little Dance by Kisnau. The fic is a FrostIron story.