You don´t have to use this form if you don´t want. In portuguese you can also say " as crianças são boas estudantes", because the adjective can follow the genre of the both subjects ( estudantes or crianças ). By the way, as a native speaker, I always use the form that you think as a right way to say ( boas estudantes). Actually i´ve never heard somebody speaking "as crianças são bons estudantes" it doesn´t sounds natural .....
This I don't understand. Why do I have to put "bons" if "crianças" is feminine?
Because bons is related to the word estudantes, that is masculine in this case, and not to the word children. "Estudante" can be either masculine or feminine.
This is not correct. Estudante is a both feminine and masculine word. We call it "comum de dois gêneros". So the right answer should also be "As crianças são boas estudantes", because you could be talking about girls only.
We brazilians translate crianças as ''kids'' in a way that we simply don't wanna mention that they're sons or daughters of some parents. Just for saying that they're young. ''Filhos'' refers more to the fact that those children, people are Sons or Daughters of some parents. Not saying that ''Crianças'' (kids) don't have any parents, but simply because ''Crianças'' can be used in a general way, to refer to those young happy and active miniperson.
Hm... I don't know... they have a word called ''Gajo'' but I don't remember if means the same as ''Criança'' (Kid) or ''Filhos'' (Children) or even Boy/s. But Keep up with the work, I'll be there helping you.