I guess there is a logical explanation as to why the sequals took such a childish increase. I remember Malte said that there can be gloomy elements without completely affecting the younger audiance. While I do agree with you on this, I think the first movie might have went a little too far in certain areas, which is probably what caused certain scenes to be cut. Scenes like the sharptooth fight inthe beginning, Littlefoot's mother's death, and the sharptooth encounter at the end, were probably seen as "too much" for the younger audiances, and maybe people complained about it and that's why the sequals are nowhere near as dark as the first one. Then again, I think we can all agree that the first movie was successful because it emphasized the difficulites one may have to go through in order to find peace.
Having said that, I really enjoy the sequals. I think these actors are really talented and have a really nice career ahead of them.