I think that there has been a lot of valid points made in this discussion already regarding tackling at such a young age.
Personally, I agree with those who have already stated that the sliding tackle is, love it or hate it, a part of the game. For defending players I think it would be a huge disadvantage for them to have a technique that they will use later on in their soccer careers unavailable to them when they are starting out. The underage game provides an invaluable opportunity for players of all levels to perfect the technical aspects of the game that are an integral part of any senior level player.
I do think, however, that the real problem here is the parent. In my brief time working at a pro club, as an academy coach, I've never been so shocked as I was when mild mannered moms and dads mutated into seething, vicious animals on the sideline... whether its "encouraging" or "advising" they are ALWAYS doing more harm than good, whether its by merely distracting their kid from the game by addressing them by name, or worse, by openly criticising their own kid (or someone elses)... parents would be well advised to just let their kid enjoy the game first and foremost, and then, let the coach do the coaching.