Kid and child should be interchangeable for "đứa trẻ". If your answer is not accepted, please click the report button.
Additionally, "đứa trẻ" can mean kid who is either a boy or a girl, at very young age. The reason we do not accept "boy" for "đứa trẻ" is because in English "boy" does not have a specific range of age. A 16-year-old boy is a "boy" but it's not the case in Vietnamese.
it doesn't refer to the same person. when I say "đứa trẻ", I am talking about a child, quite younger than me. I wouldn't have used "đứa trẻ" if I was a fellow child myself. "anh ấy" is someone of my age or a bit older than I am. I don't know why they put those together in the same sentence, confusing people who are not familiar with the language.
I like your explanation! The english equivalent (answer) doesn't make this distinction which means translating back would be problematic. I suspect this might be included here to introduce situations where subject is important in vietnamese such that the direct english translation is ambiguous. In fact the normal flow of english on this direct translation would suggest the child (which can therefore be male only) that studies/learns is the one drinking water which is not correct. One can only hope other topics or future learning addresses this or makes this inconsistency clearer.