A bit difficult, but still a fantastic game
I absolutely love this game. It's among some of my favorites for a lot of reasons, namely it's story, art, and gameplay. This story is full of great messages and role models, so it could work for kids.
Note that I said could, because as many others have said, this game is pretty hard. The controls are easy enough to remember, but obstacles are pretty hard to avoid, you have to have pretty fast reactions to get past them and even with that in mind, you could still die literally HUNDREDS of times (seriously, you should see the leaderboards, some people have died up to 700+ times). Oftentimes, your objective won't be completely explained and you'll have to get past plenty of puzzles that don't have very clear means defined. Autosave is also pretty scarce, so unless you remember to top off your energy and use it to save, you could get set back pretty far. That being said, the difficulty makes the game INCREDIBLY rewarding, persistence is rewarded thoroughly as you get to experience some truly unforgettable moments and eventually you begin to truly master the game, you also get to explore some very interesting areas you couldn't explore earlier (similar to the metroid franchise).
Going back to the positive messages, this game shows that forgiveness and compassion are important, even towards one's enemies. For example, Ori is a very selfless character and his quest is motivated by the desire to save others rather than out of hatred against Kuro, who practically ruined his life. Naru is also a very caring and compassionate character who shows motherly love, not only to Ori, but again, to Kuro and Kuro's children. Gumo starts off selfish, but it's forgive able since he's the last of his kind, and he eventually does more than enough to redeem himself by helping others like Ori and Naru do. This emphasizes the message that all life is sacred and precious, one should not take it for granted. But it's not so forceful with it's message like so many shows and games even are nowadays, it's very subtle and shows it in a more realistic light, by showing the bad as much as the good so that it feels more real and persuasive.
So if you're looking for a great game for your kids, even one like this that sends a great message in an effective way, I definitely recommend it. Sure it's difficult, sure it takes a while to master, which kids aren't very used to in a lot of games, but dang it is a great game. Just be sure your kids are ready for the difficult journey that lies ahead, it'll teach them about persistence and strategy in addition to it's aforementioned messages.