I loved this movie!
I went to see this movie recently with some friends and loved it. I would not suggest for children younger than 10 or 11 years of age to watch it, but parents know their child better than anyone (except the child themself, probably), and I believe that the parent(s) should be capable of assessing whether their child is mature enough to watch movies like this. The violence level, in my opinion, really wasn't too different from that of the Star Wars movies. You do, however, need to be able to pay close attention to the plot and have a decent amount of knowledge about the Harry Potter universe. I would say that if your child is mature and has had prior experience with watching darker, more violent movies, The Crimes of Grindelwald is an excellent one to watch. Note, however, that it is called The Crimes of Grindelwald for a reason, and may not be great for very sensitive kids.