Hawaiian Mickey and Minnie are adorable! In order to make this happen I would suggest locating the daily 'Iwa or schedule to see what times the characters are appearing. The schedule is located in various spots in the lobby. The characters seemed to congregate on level one in the pool area closest to the 'Ewa tower, or just below the lobby to the left hand side if you are facing the ocean. The times seemed to be early in the morning for the character breakfast(and only for those eating breakfast) or later in the day between 10:30 and 12:30. They are all at the Starlight Hui show but cast members were asking for room cards or to show your daily bracelet for admission.
To avoid all the trouble, I would schedule a breakfast. I heard it is a great experience. Good luck!