With the release of
Return to the Clans the Tadpole mystery has been solved. I got to flick briefly through the manga and when doing so, I discovered why Tadpole was only mentioned a few times...
Warning! Spoilers for Return to the Clans!!
Sasha tells her kits about her past two-leg owner, Ken. Hawk, Moth and Tadpole decide to try to find Ken to make their mother happy. What they don't know is that Ken had grown very old and gone to a nursing home, far, far away. (That is why Sasha became a loner--originally she was a kittypet.)
When Sasha goes out to hunt, the kits wander off and find BloodClan warriors. They run away and discover an abandoned two-leg nest. The kits then go inside the open basement window but accidentally seal themselves in. A pipe in the basement bursts and water begins to fill the basement.
Sasha, meanwhile, has returned to find her beloved kits missing. At once she begins a search for them. She encounters the same BloodClan warriors that her kits do. She manages to find the basement and rescue Hawk and Moth, but little Tadpole drowns.
The same night, Sasha dreams of Tigerstar. She asks him if Tadpole is with him. Tigerstar says no, but he tells Sasha that Tadpole is safe. This must mean Tadpole went to StarClan.
In the end, Sasha takes her kits to RiverClan then decides that Clan life is not for her. She leaves, though her kits stay.
And that is what happened to Tadpole. The poor little kit. :(
-Sorrelpaw
One last thing...Russetfur knows who Hawk and Moth's father is...