[RadioKid]

>Am I pretty?
>Watashi kerei?

Very good! These words are often talked in such casual scenes.

If you want to say "Am I very pretty?" it would be translated as "WATASHI SUGOKU KIREI?". But it is not natural but arrogant. If you want to emphasize her character, you can say "WATASHI KIREI DESHO?".

>Come meet me in the desert.
>Sabaku de watashi ni ai ni kite.

How about "SABAKU DE AI MASHOU"? This wuld be as natural as "WATASHI KIREI?".

>I'm under the sand.
>Watashi wa suna no shita ni iru.

WATASHI WA SUNA NO NAKA DE MATSU.

In Japanese, "under the sand" is translated as "SUNA NO NAKA:in the sand".
Also "under the water" should be translated as "MIZU NO NAKA;in the water".

And if you don't mind, write romaji in capital case like as "WATASHI" because it is confusing when romaji and English words are mixed.