One of my kids used to be too far back (not as badly as the child in the photo). An instructor cured the problem in one day by having him go to a bump run, stop on to of each mogul, get forward, and then turn. Once he skied more normally (linking turns w/o stopping) he was more neutral or forward. His better balance, of course, led to better skiing and improvement.
I guess the stomach and leg strength of young kids (3 to 6) is also an issue. So are the boots, which are usually made for comfort. They may not be stiff enough for the little Lindsy or Bode to feel resistance from pushing forward a bit. I never thought my little guy being a bit in the back seat was a big deal since he was having fun and I assumed he would figure out how to get forward soon enough (he was OK by age 7). Besides, he isn't going to be earning FIS points anytime soon.