Not one to casually chuck out the "Childhood ruined" meme, but it is hard not to take the butchering of a treasured childhood story personally.
In fairness, you can see the thought process behind the production decisions. The renaming of Titty is a meeting I feel like I was present for it, is so easy to visualise. The addition of the international intrigue angle adds drama, and you can see a producer demanding it: "you want to make a movie where a bunch of wholesome middle class white kids sail round a lake for a week and then have a tea party? Where is the action? Where is the tension?" I can imagine the script writer going back to the source and coming across Arthur Ransome's Russian connections on wikipedia and having a lightbulb moment that powers them through a midnight rewrite session.
All this I feel could have been forgiven, and even set a little differently to a much better result: The movie could have been somewhat more meta: with the spy plot line going on in the background and the children oblivious to it. Its actually possible to re-read the events of the book as exactly this, with Uncle Jim's various activities made ambiguous, yet the events as perceived by the children remain untouched. This could have added that layer that a scene by scene retelling of the book would have needed to make it compelling on screen, but would have retained the original's charm.
The unforgivable sin of this movie is the degradation of the characters. Not one seems un-touched by a 21st century cynicism: All the adults are hostile, with the generational dynamic speaking more to that between baby boomers and millennials than the values of the 1930s.
The Blacketts have none of that self aware joie-de-vivre. In the books, Nancy is full of bluster, but always reconsiders when about to behave unwisely. In the movie she comes off as casually abusive, hostile and borderline a bully, becoming an ally to the Walkers in the final act with no apparent resolution to the earlier tensions.
The Walkers grieve me the most. They bicker and are incompetent, when in the books they are committed to performing practical tasks of camping and sailing diligently, capably and considerately, always eager to absorb more knowledge and skills and consumed by the learning of their crafts. This was such an inspiration to me as a child, and to see such fundamental carelessness from these loved characters truly grieves. John is abusive and a poor leader, written so purely to set up drama and conflict. Thrown by the wayside is his simple decency from the books, as well as the most interesting struggles with the expectations of his absent father.
I don't want to be unfair to the child-actors, but their performance is frequently painfully wooden, which I'd actually lay at the feet of the direction rather than the actors as even the adult leads fail to shine. Andrew Scott in particular is interesting to contrast with his role as Moriarty in Sherlock, where he is compelling every moment he appears.
For the most part the child actors are debuting in the film. I hope it doesn't stand as a black mark against their futures. In particular Dane Hughes (John) has to be given credit: I might hate the script, character notes and direction he was given, but he gives it his all and adds some believable nuance to his scenes that spoke to me even while I was hating the movie. Teddy-Rose Malleson-Allen (Tatty) also shines: her performance is easy and natural, and overcomes what should be the cringe factor of several of her lines by a number of notches with an innocent charm. Overall poor then. To be sure the source material being very difficult to simply translate directly to the screen is a hard starting place, but several bad decisions compound things, many of them being common irritating features of contemporary film making in needing to add tension and action. I feel as a fan of the source material that a good movie, retaining the innocence and themes of competence, duty and loyalty, is out there if better choices were to be made. Additions to fit the screen medium are of course necessary, but not these additions. Sadly I fear we may not see another attempt to make it work for another 43 years.
