I don't have any particular critiques of this book, and I appreciate more representation of Black and brown trans boys (this is also a mixed-race family, with a Black father and a white mother -- matching the authors, I realized when I looked at in the inside back cover flap), but I also feel like there's not much to this book.
We open with, "For as long as I could remember, I knew I was a boy." Our child protagonist tells us that he would draw and imagine himself with hair and clothes like his father and brother, and we then quickly move to him deciding it's time to tell his family. I'm fine with the book just taking for granted that Calvin's self-identity is legitimate, rather than taking time (as other books understandably do) to note that girls can enjoy activities, clothes, and hairstyles associated with boys.
Calvin's dad does say, "We love you if you are a girl, boy, neither, or both," and teaches Calvin the word "transgender" (with a definition I don't love, since it seems to imply that being trans means people will always get your gender wrong -- "Being transgender means other people think you are one gender, but inside, you know you are a different one" -- but I understand what it's trying to get at).
Everyone is very supportive, which is nice, albeit somewhat unrealistic. They are about to go on a family trip when Calvin discloses, and his dad tells his relatives (who I think are the father's parents) his new name, and they're fully accepting. When school starts up again in the fall, Calvin's new name is on all the appropriate spots in his classroom -- and his dad has told at least one of his friends (I assume the scene with her is intended to be a stand-in for all his friends, though it's a little bit weird that Calvin's dad didn't tell Calvin he had done that) as well as the principal.
[Edit: We don't actually learn Calvin's new name in the text until the very last page -- which I hadn't realized until I read the book to my partner (who wondered, in the first classroom scene, whether our protagonist's name was "Calvin" or "Alex" -- in fairness to the book, "Calvin" is the only name that shows up in every place we see namecards, but my partner understandably most noticed the cluster of desks that is somewhat visually centered) both because I was primed by the title etc., and because the book is fairly artful in its avoidance. Because I felt like I already knew our protagonist's chosen name, the dramatic reveal at the end was a bit lost on me.]
Calvin's worry recurs a few times throughout the book, along with his father's advice to "take a deep breath and count down from 5," which is some nice modeling for the child-reader.
There's also casual diversity in the book throughout the book. The family they're visiting is Black; and when Calvin gets to meet his favorite superhero (also Black) at a comic convention, the background crowd includes fat people, other Black people, someone I read as an East Asian woman, and someone I read as a hijabi (though that may have been unintentional with her costume). The principal and the one friend Calvin has a conversation with are both coded as East Asian, Calvin's classroom teachers are a Black man (with long dreadlocks) and a brown woman, and the student body is very diverse (including one child with vitiligo and a wheelchair user -- who are both somewhat highlighted in the closing playground image, holding the jump rope while Calvin and Violet jump -- as well as some fat kids).
The Authors' Note indicates that the authors have a transgender daughter, so I thought it was interesting to make the protagonist of this book a transgender boy. Maybe they didn't want people reading it too autobiographically? (They do note that, in writing this book, they've drawn on the experience of "countless trans kids thriving when they're supported by the adults in their lives.")
They don't say anything specific about why they chose to write this book, just, "We know that talking about these topics with children can be tricky, especially if you're learning about them for the first time. That's why we we wrote this book."