Lots more spoilers.
The werewolves are easy - just make them adults - but the hags will take some thought.
So, reading more of the book, the werewolves and the hags appear to be the only other places where there are dead children. The drunk guy on the lake is trying to kill a child, but it hasn't happened yet (the other places have specifically lots of dead children around, including bones and clothes and it's really obvious). If I find more I'll post, or if anyone knows of others please say. But I'm happy to open this thread up to those situations as well if anyone is interested.
The difference is that in Death House, the dead children ARE the plot. In these other places, they are flavor (bad word choice) - window dressing. Much more easily changed.
In the first case, the werewolves can just have kidnapped people, not children. Very easy change. There could still be the fight to the death stuff, but the victors are adults and then they get turned. Secondarily, if you keep the werewolf children, they could be just literally that: werewolf children, the offspring of werewolves. This opens up whole new moral questions for the party, as well as changes werewolf lore: normally (like vampires) werewolves don't age and don't reproduce except by infecting others. It also calls into question why they would even have the battle royale, since they can make more of their own. Maybe they aren't typically able to reproduce or maybe they're dying too quickly and need more recruits more quickly. Maybe they're just evil and they like it. Or maybe the two factions are also arguing over the "natural born" werewolves; the current leader wants to abandon them in the woods since they aren't viable soldiers and the woman wants to save them since they need the recruits long-term. CoS already messes with werewolf lore by making children into werewolves and then having them grow up. So you can either go wild, or just completely edit the werewolf children out entirely, which is also extremely easy. This changes absolutely nothing and makes the whole situation much more morally straightforward. Still, "how to deal with werewolf children" should be interesting; at the very least, it puts the "how to deal with children who just woke up after 200 years" question to shame.
The drunk guy on the lake can be modified to be dumping an adult woman. Literally nothing about the situation changes except the person ages ~20 years. I'm not even sure why they made this a child anyway, other than to make it more plausible that an old drunk guy would overpower her. Or you can change the situation depending on what happens; if she is rescued, keep her as-written age. Depends on how close you want to get to the line. (You could also make her a young man instead and it would have zero impact.)
The hags are a difficult case because while there's no reason you couldn't age their victims, this goes more against the typical Grimm's fairy tale lore of old witches eating children. I'm not sure how else to fix this, though. In this case, the hags wouldn't be getting parents to pay for their drugs via children, but they'd be either: taking the person themselves (seems less likely and also goes against the nature of what they're doing - allowing people to damn themselves eyes wide open), taking someone else in their stead like a spouse (they could force the addict to ensnare the spouse), or telling the person they would have to come work for them - but really they capture the person in a cage when they get to the windmill. The last option makes it slightly less likely that the party would intervene when they first see a hag taking someone off to "work for her." I mean, she's just following a contract she made with this individual, right? The fact that no one ever returns might pique their interest. Forcing people to sell off their spouse or aged parent or something (against that person's will) seems a little closer to the original idea, though. Seeing a hag take a human-shaped bunch of rags from someone should also get the party investigating.
You could also go a more drastic route and just nix the drug addiction aspect of the story altogether. The hags just live out there in the windmill, capture anyone who comes by (maybe the treats just knock you out), and eat them. (They could still have a couple of captured adults in their cages.) Much more bland story-wise, but simple enough to change. If you wanted to spice it up you could change the windmill into something more appropriate, like a cottage made out of candy. Of course, no player is going to fall for that.
Either way, you can change the piles of clothes to be adult clothes and the teeth at the standing stones to be adult teeth. Really anywhere you have piles of things that imply dead children can be changed without any impact.