Translation:This is my new book about boys and girls.
This is absolutely true and it can be seen as either an issue or a feature with multiple courses on Duolingo. The words “мальчиках” and “девочках” in isolation can either be instrumental or prepositional plural. The instrumental case lesson teaches these two words because they can be in the instrumental case and so Duolingo places them here. It can be confusing, but it can also help you make connections and associations within a language that are not necessarily grammatical but still correct.
True, "a book about" something or someone is more common in English, but "a book on" something or someone is also quite possible (with "on" having the same meaning as "about").
"A Book on Life and Death Becomes a Best Seller" is an example headline in a major American newspaper. https://www.wsj.com/articles/a-book-on-life-and-death-becomes-a-best-seller-1482167354
Books "on" particular subjects are not the same as books "about" those subjects. Books "on" any particular subject usually are discussions of the subject matter in a disassociated format - objective rather than subjective, distanced rather than involved. For example, sociologists would read books "on" the relationships between boys and girls at various ages. Novels dealing with the same subject matter would be "about" boys and girls; there is no distance between the reader and the subject matter, if the novel is any good.
"Adolescent society: The social life of the teenager and its impact on education." is a book on teenage boys and girls. One could say that this book is also "about" teenage boys and girls.
"Romeo and Juliet" is a play about a teenage boy and girl. One could not say that this play is "on" two teenagers.
I completely disagree, as words can have more meanings than one, depending on the context. In most cases you can infer what "on" means from the context and that "I'm writing a book on boys and girls" can have the same meaning as about, though to avoid confusion I would use about here too.
"A book about the boys and the girls" sounds very odd. I can't think of a scenario where a native would say it like that. Even if you were refering to a specific group of boys and girls, you would say "the boys and girls" (leaving off "the" before girls) but you still have to give context for the specificity, like "the boys and girls of Smith Elementary".
To my feeling, there's a slight difference between the two. 1 x the = a book about all children as a whole, their interactions, etc. 2 x the = a book about the boys on the one hand and the girls on the other. Could be different chapters for both, or not, but it implies a segregation. This may well be an interpretation, but it feels correct to me.
Could someone explain the distinction please. I thought the word это was supposed to be used with neuter nouns and that in thia case where книга is feminine that they would write эта. (If i did not understand correctly, I also think этот is the masculine form) or do they have completely different meanings
Это has two meanings:
1. As you correctly say, it is the determiner "this/that" which is attached to nominative neuter singular nouns: это место "this/that place"
2. When not attached to a noun, it means "this is", "it is", "these/they are". In this function, "это" is invariable; it does not change, even if associated (but not attached to) a particular noun:
Это мальчик "This is a/the boy"
Это девочки "These are girls"
Это места "These are places"
This is my new book about boys and girls. It is different from my other books about boys and girls in that the boys and girls have different names and that the actions are in a different order. If you liked my previous books, you will definitely love this one.