Like many other ideas, this sounds good until we consider the reality.
My own keriyah is excellent, and I am able to read Hebrew very quickly. I am highly familiar with Aramaic, and I understand and enjoy reading Targum Onkelos, which indeed enhances my understanding of many pesukim.
On a typical week, if I’m in a rush, I can complete shnayim mikra v’echad targum in about an hour for the entire parasha (without Rashi).
I am also a Rebbe in an upper elementary grade; I know my own students, and have done some informal research on the keriyah level of students outside my own community.
The fact is that most students would be overwhelmed with these assignments simply because their keriyah, even if it is accurate, is not as fluent as that of most adults. This means it would take them significantly longer to read even smaller selections. Add to this the fact that they do not understand Hebrew (let alone Aramaic), and nearly all elementary students would quickly become frustrated and fatigued by this task.
I am well aware of the theoretical benefits of such homework, however the reality is that for the vast majority of children these benefits will not be realized at all. Instead, students will waste whatever appetite they have for learning on meaningless and arduous exercises in futility.
If your son is one of the few exceptions, by all means, encourage him to do this – the school, however, must act in the best interests of the most students.