Heiankyo Alien. It was the cheapest game in the store, so that's the one my parents let me buy. For a long time it was one of only a couple games I owned--that one and whatever it shipped with... Tetris?
Trophy Loom is a setting book. It's not required; just like you don't need a Forgotten Realms or Dragonlance book to play D&D 5e.
Trophy Dark is a system for one-shots. The book includes a bunch of adventures. The PCs are expected to be irrevocably changed, "lost" to the horrors of the world, or dead by the end of the session.
Trophy Gold is for campaign style play. The PCs are more likely to survive and return to town at the end of a session. There's a few more mechanics than Dark, but both are rules-lite. The book comes with a few adventures.
The tone and genre for both games (and the setting book) is dark, gritty, dangerous low-magic, late-medieval/early-renaissance fantasy.
Edit to add a content warning: horror is a main theme. And body horror is a big part of many of published adventures.
It's mixed at the tap. At a soda fountain thw water, CO2, and syrup are stored separately and mixed at the tap. Flavor difference is likely the result of a different ratio.
Just understand that playing a game with kids that young means throwing out the rules as soon as you start. So you need something with absolutely minimal rules (or you're wasting you time) and a story that's easy for them to latch on to.
I love using Honey Heist with kids. It's basically Lasers & Feelings, but you're a bear disguised as a human trying to steal honey from humans. Kids can really take that premise and run with. If you run multiple sessions do it picaresquely, where each session is a self-contained episode.
I've tried a few for ios and landed on Bean.