tpham:
Welcome to the forum!
MrLuke has given you excellent help; the only answer, actually. Essentially, the iPad is designed as a "one user" device. So, there's truly no way to get a password put on the Mail app for your account(s).
Note that I have made your post a new thread within the Help section of this forum. You had posted your question (completely unrelated) in another person's conversation, which is poor forum etiquette and against the rules. Thus, this move to your own thread. Please, for the next time, start a separate thread for your question/concern, don't interrupt others. Thanks.
Sorry we can't give you the answer you were seeking on the password question.
Marilyn