Chinese characters do not work the same way that English words do.
Since Timmy is an proper name, there would not be a direct Chinese
Traslation. What the Chinese would do is find characters that
would make the equivalent sounds.
Of course, on a tattoo, not knowing it was a name, it would just be jibberish to anyone who could actually read the characters.
If they would like to have two characters that when spoken aloud make
the sound timmy, I can accomodate, however, it will not make much sense.