What does it mean that most readily available children’s books about aliens feature them as a evil invading armies rather than curious individual explorers?

One of the psychological elements of warfare is dehumanizing the enemy. So in this instance, the author’s work is already done if the enemy isn’t human to begin with.

Or that those books that do include arguably harmless aliens generally characterize them as ugly, gross or stupid?

Simple… aliens are the new minority–the only politically-correct ones at that. They are the latter-day blacks, immigrants, “injuns”–pick your favorite underclass out of American history. Except there’s been no Alien Civil Rights Movement to put an end to the abuse. So aliens are the meta-underclass, the universal inferior. Even “Toy Story” ran afoul of this with the cultish, isolated, and naive aliens from the Crane Game. So…

I don’t think we learned much from all those Star Trek re-runs.

Daniel Pinkwater’s Mush, A Dog From Space. As you might expect from Pinkwater, it’s funny as all get out.

Seriously, I find this shocking. Typical in the current political and world climate though. Have you checked publishing records and plans before 9-11 to see if there was a radical change? Wouldn’t surprise me. But if it happened, it’s all the more reason to start hunting for something new!