Note, July 26, 2019: I've just edited this review to correct a chronological error --thanks for pointing it out, Shannen!
Although this is the second novel of Alcott's Little Women trilogy (Part 2 of Little Women, the first novel, was first published separately as Good Wives, but after that, the two were published as a unit), I read it first, and at about the age of eight; it was one of the earliest books I read by myself that I can actually remember. (As I sometimes say, I "cut my teeth" as a reader on Victorian and Edwardian-era classics.) This review has no spoilers for this book, but the situation it describes inevitably involves some "spoilers" in relation to the preceding book.
At the conclusion of the previous tale, Jo March and her German-born suitor, Professor Bhaer, are engaged, and planning to turn Plumfield, the country estate outside of Boston that Jo has inherited from her now-deceased Aunt March, into a boarding school for boys. I don't have Little Women in front of me; and don't perfectly recall the conclusion, but at that time they were planning to marry the following year. The second book simply recounts about six months, from spring to Thanksgiving, in the life of the school --an eventful period that introduces several new pupils-- beginning when their oldest natural son is a bit younger than his twin cousins, who are 10. (That sets up an interesting chronological situation; the internal chronology of the first book, which was published in 1868-69, means that the Bhaers couldn't have married before 1870. This would date the events of Little Men no earlier than 1878, when the cousins, born in 1868, would be about 10; but it was published in 1871. So Alcott was projecting the events, from her own perspective, several years into the future. However, the real-life material and social culture didn't change markedly from 1871 to 1878, so the text as we have it fits pretty well into that chronological setting.)
At one point in this book, Alcott writes "...there is no particular plan to this story, except to describe a few scenes in the life at Plumfield for the amusement of certain little persons...." As this suggests, it was written for younger readers; and I think it has a slightly less "grown-up" tone, and deals with somewhat less (or deals somewhat less with) serious and deep themes than the first book. (That may be simply my impression from reading it myself at a very young age, but I don't think so.) It also correctly suggests that there's no very intricate plot here; though the story-line is eventful, the book is somewhat episodic, and more a study of characters than a plot-driven work. It's also an illustration, by rosy example, of the "progressive" educational theories of Alcott's own father, Bronson Alcott, who served as the model for Professor Bhaer; because Plumfield is an unconventional school in a number of ways. Personally, I think a lot of the ideas used here really do have merit. But I'm very skeptical (and already was as a kid) of his rather pacifist approach to discipline --rather than him using the ruler on the hands of offenders, for instance, they have to strike him with the ruler. (Alcott's father actually used that technique.) IMO, it works a lot better here than it probably did in actual practice, and Plumfield is a more successful school than any of Bronson Alcott's real-life educational ventures really were. Unlike Jo, Alcott herself didn't have any actual experience with running a school, and tends to view the kids in the book with somewhat rose-colored spectacles; they don't generally present many serious behavioral issues. (Though to be fair, there are some of these, especially surrounding one of the boys.)
All of that said, there's a lot of realistic incident here, and very good development of character; the dozen or so boys at the school, and a couple of girls --Jo's niece Daisy attends Plumfield with her twin brother, John Brooke Jr. (hence "Demijohn," or "Demi" for short), and tomboy Nan winds up here as well-- are all developed as distinct individuals, and drawn as vividly as the adults. (Two of the newcomers, musically-talented Nat and rough-edged Dan, have the most of an actual story arc associated with them, and Dan is the most dynamic character, in the sense of growing and developing in the course of the book). I liked most of the boys, and both of the girls, but Nan was far-and-away my favorite of the latter (I guess I had a thing for tough tomboy types even then, and she earned my admiration early :-) ). Childhood friendship, good life lessons for growing up, adventures, mischief, puppy love --it's all here, and Alcott tells it well. The diction isn't hard to understand, even for kids (at least, motivated kids who like reading), but the story and story-telling isn't so "kiddish" that adults couldn't enjoy it. In fact, I've decided that this would make a good book for Barb and I to read together sometime! (I'd normally recommend that one read Little Women first; but in her case, she saw and liked the movie adaptation of the latter.)