I think Sora and Riku are mentioned in Birth By Sleep. If you look at the bbs trailer there are two boys fighting with wooden swords on a beach. I think those two are young Riku and young Sora, but I'm not so sure.

Yup! The two boys fighting with wooden swords on the beach is Riku and Sora!

mebmer13fan is right!

sora and riku in kingdom hearts birth by sleep

I know, I did that one a LOOOOOONG time ago! Sorry.. <:'

Sora's and Riku's name was mentioned as well as appearance in KH BBS, look it up on 1up.com. Mickey and Kairi along with most of the org XIII somebodies where also seen

yeah sure...

Yesss! Although.....
--Spoiler--
In Birth By Sleep, Ventus is dies and his soul finds Sora, as a child, in the Darkness. Ventus allows Sora to absorb his soul so that Ventus can be reborn into the Light. Eventually, Sora finds/becomes Light. (No, Not Light Yagami. Stupid Turds!) After Sora loses his Hearts/Releases his Heart to give Kairi her own back. Roxas was inadvertently created. Roxas may look like Ventus, but in reality Roxas IS Ventus.
--Spoiler Ends--
*In any of this info is incorrect, PLEASE CONTACT ME!!!!!*

NO!!im just kidding! Roxas IS A MAIN CHARACTER!!!!(hes Ventus)

And, fyi, Riku and Sora are (not of their knowing) being judged by Terra and Aqua to see if they, or one of them, are worthy of being keyblade masters....so....yeah theyre in it too...and Xemnas...Xigbar, Xaldin, Vexen, Lexaeus, Zexion, Saix, and Axel. When they all were Somebodies...*possessed or not*

Sora's four in it, so he's in it

KH3D is Sora and Riku's M of M Exam by Yen Sid.

they already are