The film really focuses on death and grief- dead superheroes, dead parents, grieving fiances, grieving husbands, grieving parents. The F word is used precisely 3 times- once by a villain and twice by heroes- this is not a high-profanity movie, but it's there.
A body is zapped out of existence by an alien weapon and you can see bones and muscle being vaporized in slow motion but not bloody, more like an anatomical illustration. The main CGI villain gets decapitated at the end, bloodlessly, after a pretty violent pummeling by various superheroes. I recall seeing a VERY small amount of blood in a couple of battle scenes. The violence is mostly large "Return of the King" type battle scenes of CGI aliens or warriors being stabbed or zapped. In terms of values and messages, it emphasizes the importance of teamwork, cooperation, self-sacrifice, responsibility and heroism, making amends for one's mistakes, helping those who need it, and showing compassion for those in pain and dealing with loss and grief.
This cut is generally an improvement over the 2017 cut- more time for character development and more superhero action. Individual scenes in Justice League might be fine for younger kids (there are some very good Wonder Woman and Amazon war scenes that I would say are fine for younger children than that but as a whole the film is very murky and dark and grim in tone.) I honestly think most kids would be bored more than scared, due to its slow, mood-setting, character-building pace. I love showing superhero movies to my kids but the DC movies of the past 10 years are usually much darker and more intense than the Marvel films- so the lowest age I usually recommend most of these for is kids 12 and up- I've made some exceptions here and there. The 2021 version, a 4 hour long re-edit by the original director Zack Snyder- (Not the 2-hour 2017 "Justice League" film started by Snyder but completed by Joss Whedon. To clarify, I am writing a review of the "Snyder Cut" of Justice League.