YOU SAID:

her forehead sit on my navel

INTO JAPANESE

私のおへその上に彼女の額座る

BACK INTO ENGLISH

I will sit on her belly above my belly button

INTO JAPANESE

私は彼女の腹の上に私の腹のボタンの上に座ります

BACK INTO ENGLISH

I will sit on my belly button on her belly

INTO JAPANESE

私は彼女の腹に私の腹のボタンに座ります

BACK INTO ENGLISH

I sit on my belly button on her belly

INTO JAPANESE

私は彼女の腹の上に私の腹のボタンの上に座る

BACK INTO ENGLISH

I sit on my belly button on her belly