The history of Ravenloft under WotC has consistently been "gothic horror", with no modern horror explicitness. From I6 Ravenloft and I10 Ravenloft II in 1e, to Domains of Dread and Expedition to Castle Ravenloft, the setting is "dark" in the sense that Hammer Films were dark, but if a minor can handle that kind of media they will be fine with Ravenloft. Actual play reports claim the same for the new 5e Ravenloft.
I will also note that WotC has always had extremely strong content codes applied to their works in general and Organized Play in particular (when I was a Living Greyhawk triad, an Erik Mona adventure got censored because of the implication that some critters had hurt some children). I think it's safe to say you have nothing to worry about, they have kept their stuff consistently PG over the last several decades. The Adventurer's League Player's Guide has a Code of Conduct in it, the campaign doesn't allow NE/CE alignments, etc.
Let me start off by saying that Curse of Strahd certainly has some very challenging encounters, many of which can be stumbled into at points when the party is woefully under-levelled. I've only run it once so far, but that did end in a TPK.
In Vallaki, the party has the opportunity to run into a nest of vampire spawn whilst investigating a coffin maker's shop. The coffin maker (at least under my interpretation) is being kept as an unwilling servant/hostage of these vampires. I tried to give as many hints as possible that the coffin maker was scared of something in the shop - he was glancing over his shoulder, refused to let them in, would only speak in a whisper and asked the party to meet him later 'somewhere safe' suggesting the inn. Unfortunately the party didn't take the hint. Instead, they separated, two distracted the coffin maker, whilst the remainder broke into his house. I tried to give further hints - letting them spot many footsteps around large body sized crates in the dust of the storage room. I even pointed out that the cleric in the party felt something deeply disturbing from the room. Irrespective, he cracked open the crates, triggered an enormous fight and ended up dying.
It's important to note, however, that this type of danger is a common feature of sandbox campaigns. On top of that, this is a horror campaign - it is not unreasonable that if the characters poke around the dark corners of Barovia whilst uninformed of what lies within, they may end up angering something much more powerful than themselves. I personally feel that without such a threat, the player paranoia and tension necessary to maintain an effective horror theme can end up being lost or diminished.
Players should be made aware of this when starting the campaign and they should note that often discretion will be the better part of valor. It also behooves the DM to provide more narrative warning in such cases - signs of danger, direct warnings from NPCs, and (for when it all goes awry) avenues of escape. Even with this, however, encounters can and should have the potential to be fatal. At that point it is down to each DM how such an encounter plays out - nothing precludes you from granting strokes of luck, fudging rolls, or throwing in timely ally arrivals when things go south. Only you know your players and the game you are running, hence it's up to you and what you most feel makes for a fun game around your table.
With regards to your specific examples:
Death House - this is a hard starting adventure, but one that I found to really set the scene and vibe of the campaign. The shadows at the statue, the two ghasts, and the shambling mound stand out as all potentially being fatal encounters. The shadows, however, are extremely avoidable and are essentially a consequence of a lack of caution. Much of their threat depends on the party composition and their ability to output magical damage. Contrary to the book I would probably not have the shadows pursue the party out of the room, and may have them vanish if (for example) the orb was returned. The ghasts and the mound are tough, but certainly doable if the characters play smart.
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Bonegrinder - I feel that warnings discouraging the party from entering too early are key to this adventure. The Vistani at Tser Pool may have warned the party away from there, knowing their destination and based upon what is written on pg. 28. Additionally, the raven should be extremely vocal in trying to ward off the players. Remember, the raven will be in league with the wereravens of Vallaki and essentially a force of good. Hence, assuming the party did head in and end up in fight against the three hags, having the raven summon help in the form of a swarm or a wereraven may be one solution to avoiding a TPK.
Best Answer
In the broader cosmology (i.e. digging back to prior editions), Ravenloft is just one of many Demiplanes of Dread. Each of them is the Domain of a Dark Lord. The Domains are created by the Dark Powers as a customized prison and punishment for each Dark Lord. What the Dark Powers are or why they create these grand torture chambers is deliberately left unknown - they simply are and do.
The torture the Dark Powers inflict is based on the "crimes" of the imprisoned. Some Dark Lords have more or less power over their Domains, as it suits the nature of their punishment. Even player characters are tools of the Dark Powers. They're brought into the Demiplane as needed, used to push forward the Dark Powers' goals, and then released ("allowed to escape") back to where they came from when their usefulness ends. The Dark Powers actually care more for their chosen victim than they do bystanders. Indeed, quite often the innocent make up part of the punishment.
In Strahd's case, both his continued immortality and his pursuit of Tatyanna are part of his punishment. It is the Dark Powers that bring him back every time he is killed, despite anything characters might do to prevent it - the Dark Powers control the Demiplanes of Dread, nobody can truly challenge them. The Dark Powers use Tatyanna's soul as an implement of torture, compelling Strahd to hunt for her, find her, and lose her over and over again... so they bring that back as well. Whether it is the actual soul of the original woman, not allowed to go free (despite appearances), or a duplicate perfect enough to fool Strahd is unknown and irrelevant: it functions the way the Dark Powers need it to.