Geki, Ender's Game did age up the kids pretty significantly, and it's one of the fan's major problems with the movie adaptation. I actually liked it for the most part, they got the spirit more or less right, but it's admittedly seriously neutered from the book material.
Dpsb, I'd totally disagree with that. You don't want some horror-movie slash-fest for this, no, but it all goes to the tone, the suspension of disbelief. In a superhero movie, say Batman or Cap who're punching and kicking people around, you don't need to get graphic, that fits the tone. And people with actual superpowers, you don't need to get into the hard-hitting crunchy violent immediacy. They're fighting with laser bolts & freeze-vision and telekinesis and stuff.
With Animorphs though, the difference is that they're fighting as animals. By nature, that means teeth & claws & actual physical trauma. You could get away from that with a more breezy & upbeat tone, make it more of a superhero-y thing, but I'm sure none of us want that.
What I always loved about the books as an adolescent is that they didn't treat us with kids-gloves with the awful stuff. The battles are mostly all small-scale skirmishes, not some big "epic" heroic trumpety-music-plays victoryyyyy thing, and that each little bite & scratch & bruise hits home and feels real. There are consenquences to the stuff in Animorphs, it's not some invulnerability hero story.
Again, it doesn't have to be a hard-R bloodbath, but you absolutely have to make it more visceral & intense than say a The Dark Knight or The Winter Soldier. Not from a gratuitous point of view, but because it all seems ridiculous if that stuff isn't there, it actually effects the story and how seriously you can take it. People get hurt, people get injured, and the kids have to deal with all of that. For the story to even work, these kids have to go through absolute hell, and you're not likely to see that when there are big financial interests involved.
I'm not saying you actually show Erek walking into that building after reprogramming himself and starting to pull Hork-Bajir limb-from-limb and cave people's faces in. It's still gotta be PG-13. Just a harder PG-13 than the vast majority of the stuff we see with these adaptations, latter-movie Hunger Games or Harry Potter isn't going to cut it here.
The flick that always goes through my head as a reference point would be Terminator 2, for intensity. Obviously just without all the cussing. That thing would likely be PG-13 nowadays, if it weren't for the f-bombs, and it's both a step down from the brutality of the original but still feeling very immediate, visceral, impactful. Hits have consequences, there's real peril.
