Anything written by Gianni Rodari not only for his ideals especially evident in stories like Cipollino but also because they rock.

As a child I loved especially Favole al Telefono (telephone tales) e freccia azzura (blue arrow). But all is books are good really.

Everything brothers Grimm. They were obviously early socialists, apparent in stories such as "Handel and Gretel" which feature the obvious insinuation that the witch was an anarchist feeding off of the organisation of ML's. Don't get the wimpy versions either, gotta be the scary ones, or else the impact of such topics won't land for the children. More gore is better, you know? /s

Anti-Capitalism by Ezequiel Adamovsky is pretty cool

What age? That's an important consideration.

I have a pre-schooler, so I'll shoot some suggestions for the 2-5 range.

Lefty suggestion: Farmer Duck

It's roughly the revolutionary compliment to Click Clack Moo. Short book with cute art, and it's fun to have conversations about sharing household chores, too.

General suggestion: ¡Vamos! Let's go to the Market

First book of a fun series - they're all pretty good. I love the comic book style and it's fun to pick up some early Spanish vocabulary.

Michael Rosen is a Marxist. We're all Going on a Bear Hunt is a classic. I'm sure he published a collection called Socialist Tales, too. Could be worth a look. He narrates a lot of his stories and poems on his YouTube channel. He also wrote three books about getting kids to write. There's some good stuff in them, although it's focused on what teachers can do with young pupils. And a lot of the chapters are things rewritten from his blog. (His older blogspot blog, mind; I'm unsure if he writes on his 'official' blog so much.)

I enjoyed reading the Grufalo to my little cousins when they were that age. Julia Donaldson did one about a witch, too. Something about a cool broomstick. I don't remember the title, sorry. Superworm is great, though. There's also a Julia Donaldson Treasury of Songs, which is a good accompaniment to her books. There's a playlist of the songs on Spotify.

When they get a bit older (early teens, maybe), Mark Haddon, Curious Incident of the Dog in the Nighttime is good. It's about an autistic boy so I think it'll be a good one to subtly teach about neurodiversity. But I wouldn't see it as only that as it's a good story, too.

Isadora Moon is good. The one about going to school is the 'first' one but the author says to read them in any order. If you are introducing Spanish as LaGG_3 mentioned, this is a good one for it. I read the translation for myself and it's simple language about everyday things. Probably too complex if they don't know any Spanish. But not a bad place to start if they're marginally bilingual. Originals are in English, I think.

I'm not entirely sure what's age appropriate but the author who wrote the Very Hungry Caterpillar did a few books about: a polar bear, a brown bear, 'head to toe'. They're quite repetitive. I like them as there are bilingual editions. But be careful as the prices fluctuate. Don't get mugged! Wait till the prices come down if you want them. Maybe after Christmas would be a better time to get them. (The English editions tend to be a better price year-round.)

Please define “young kids”