In my opinion, and in my teaching, there are no shortcuts. They have to buckle down and learn positions, whether you use Simandl or Vance/Rabbath positions. On the other instruments you can play more notes in the first-learned position, and the bass kids are behind from day one. Most grade school music teachers, bless them, are at a loss how to teach the bass, which is my polite way of saying they generally have no idea unless they are bassists. If you deal with kids from local schools and have a good rapport with the teachers, request that the first lessons on the bass at school are given by you. That bit of donated time (try to get paid) may save you from having to break bad habits when they get to you for lessons. I tried that when I was still recruiting MS students, and I think it paid off.