Is it incorrect to write, "Quanto sal precisam os bebês e as crianças?"?
No, it is not. But we have to get used to writing as close as possible to duolingo' sentence in order to not lose a heart...
In Portuguese we don't change the word's position to form an interrogative, we just change the intonation, so it's better say "Quanto sal os bebês e as crianças precisam?". Anyway it's not fully incorrect. It's clearly understandable.
Lingo, it is wrong: the verb precisar needs the preposition 'de': De quanto sal precisam os bebês e as crianças?
why is the de not being used after precisar
Nonetheless, "De quanto sal os bebês e as crianças precisam?" is accepted too.
We do not end a question with a preposition.