It's not a hidden "level" but more like a hidden puzzle.
Anyway, congrats on completing Stinker Rescue! If you have done all the other levels, then you have finished Wonderland Secret Worlds!
If you have finished ALL levels with ALL bonus coins, then stick around at the ending animation after Stinker Rescue... they will show something unusual. That unusual will lead you directly to the hidden puzzle. It's NOT a Wonderland level -- in fact the hidden puzzle is outside the game entirely.
That hidden puzzle is very, very difficult... but also really short. I couldn't solve it by myself (got parts of it right), but with cooperation from another forum member I was able to solve it. It IS difficult, but it IS short.