There’s nothing quite like it really.

The closest comparison I can think of is Drifting Classroom, but it never got an anime adaptation.

Bokurano is similar in some notable ways, but it’s an ensemble and set in (more or less) our world.

Casshern Sins is sort of oddly similar - broadly similar setting and tone and a lot of the same themes, though a much different story.

I guess Noein is sort of similar, insofar as it’s a cast centered on children, the stakes are much bigger than they initially seem and tragedy lurks around every corner, but it’s different in most details.

And Kaiba is actually sort of similar too - much different setting, artstyle and story, but a similar downbeat tone and broad subject of people, and mostly children, at the mercy of a screwed-up world.

And I can’t think of anything else even vaguely similar.