Hi. Sorry it has taken some time to answer... Been a bit busy!
I believe there are only a few aspects of teaching beginners that are absolutely critical. It really doesn't matter that much what method book you are using (or no book for a while might be best), because what you get from books that I have seen is exercises, repertory, but not really good examples (other than a few photos) of form and movement. *Proper form and movement, used efficiently...just like in any sport, is the key to successful and long-term bass playing. I also believe that it does not matter that much whether you start a beginner on electric or upright. What matters is that the size of the instrument be correct or "sized" to the student. So, these three points, sizing, forms, and efficient motion, related to body shape or what I call Natural Forms at Rest, are the critical points or aspects that a teacher must be very concerned with, if he or she wishes the student to excel and have a pain-free life playing the bass. My concepts of Natural Forms at Rest and *Natural Tempos, have been discussed at length in other threads in Talk Bass and should hopefully still be available.
Another problem that many teachers come across is the availability of good quality instruments both in elec basses and double basses. *Especially for beginners who don't know if they want to pursue the instrument, investing thousands is crazy, so often a low quality, badly set up bass is used. This undermines any attempt the teacher has at creating proper form and use habits. *Remember, the first motions and forms used are memorized and are harder to change in the future (in almost every case of student coming to study with me at the U of A, I *have had to get the student to change habits of form and motion that were simply not efficient due to memorizing inefficient shapes and motions as a beginner). A beginner must be encouraged to use rounded fingers, have the double bass in an efficiently held position where he or she can reach all length of the fingerboard and be able to reach the bridge with both hands, must hold the bow with rounded and relaxed fingers with a rounded, open palm, and must swing easily both arms, in a relaxed pendulum manner, in the directions needed to play the instrument.
Nowadays, video on YouTube and such is a better way to observe proper motion and form, than from photos in method books (See my channel on YouTube:"PNeher"). But any method book gives you plenty of repertory. I would do my best, as a teacher, to integrate the sources of many bassists. With beginners I do use Simandl, but also books by the late George Vance, Francois Rabbath, Ludwig Streicher, Franco Petrarchi, Helen Stevenson, Carolyn Emery, and etude books galore by myself (of course), Sturm, Storch and Hrabe, Lee, Bille, etc. etc. etc. No one book can fulfill the needs of any beginner!!
Finally, congratulations on bringing another bassist into the world! Use LOTS of sources but put all beginners on the efficient life long track of relaxed form and motion. There are hundreds of ways to play the bass but only one is personal and will be the most efficient. The problem for many is once that approach is discovered and memorized, as the body grows and changes, it needs to evolve as well. This is where a teacher and student must be open to change and constant reassessment. I have changed my approach and technique at least three major times in my 45 years of bass playing. I am constantly evaluating my form and motion from as non-biased a view as possible, for efficiency and ease. A teacher must instill in the student the desire to be self-evaluative and enjoy the discovery, the education, and the excitement and challenge of change.
Best to you