You may remember the story about how during the plague of blood the Jews were unaffected. You may remember hearing how if a Jew drank from one side of a cup and the Egyptians the other one would drink blood, the other water. You may not have heard the verse it was derived from, but the translation is loosely, “But in the land of Goshen there was water.”
However there is no such verse. In fact the clear implication of the story is that only during the ones specifically mentioned was there no harm done to the Jews during the plagues. I think that this illustrates a elementary problem with the way chumash (and Judaism) is taught. Until you reach high school or in some cases later, there is no division between Torah and a commentator’s opinion. A consequence is that Rashi’s explanation, which was more meant for common explanation than pure scholarship, is regarded as the gospel.
I don’t think that 3rd graders should learn Ramban, but it wouldn’t hurt to maybe learn Rashbam or Ibn Ezra as a secondary commentator so that children understand that there isn’t only one possible way to understand the Torah.
This problem with Rashi is particularly accentuated while learning Gemorah. Because while you learn Gemarah Rashi is constantly inserting his subtle biases into the explanation, and, since you can’t understand the original Aramaic very well, you find it very hard to comprehend what Tosfos says.
Unfortunately the trend is not the way I would like it to be with the emphasis being on accepting one path as the only possible true one. Besides Rashi is easy to teach because he never really has controversy and you don’t really have to think. Children should either be taught Chumash straight or with more than one pshat commentary when necessary.