Part of what still amazes me about the existence of Hero Kids is a can remember when Dungeons and Dragons was getting banned at schools and libraries. And now it's okay in preschool. Wow.
And it really is Dungeons and Dragons for little kids. European, medieval fantasy with dungeon crawls. Heck, the simplest way to play Hero Kids, and the way recommended for the littlest ones, is as individual fights, effectively a small scale war game, which harms a back to Dungeons and Dragons' own origins.
Mechanically, the game is very simple. In fact, you don't even need to read to understand the player character sheets. All the abilities are in obvious icon form. Fights are Risk-style dice pools with the high dice winning when attacking and defending.
Hero Kids is one of the few RPGs I've seen with an age cap. But I think that's pretty reasonable. The mechanics are so simple that I'm pretty sure older kids, let alone adults, would be quickly bored by them.
The real impact that Hero Kids has had on my life so far is that it got me interested in reading role playing games that are designed for younger audiences. By that, I mean games that are aimed at kids before they become teenagers. Again, that such a huge change from when I first got interested in role-playing games.
I will say that, even after reading a whole bunch of them, Hero Kids still stands on its own. While much of what I read were story-driven games, Hero Kids is an old-school, mechanics driven engine, even if the mechanics are super easy.
And I honestly wonder if that really is the right way to introduce toddlers to RPGs. You see, I've seen adults struggle with improvisation and playing make believe but they understand structure and rules.
Kids, though, they got make believe down. My two-year-old, I see him playing with dinosaurs and trucks and Olaf and Minnie Mouse all at the same time. What they might need is the framework of rules. In that case, Hero Kids is perfect.
I have a couple years before my son is old enough for Hero Kids and I'll be surprised if he is actually interested in pencil and paper RPGs. I figure video games will be his RPG introduction. But, all my choices for an RPG for a toddler, Hero Kids is the top choice.