German has three grammatical genders. They're mostly arbitrary and not related to biological gender). Think of them as categories.
gender. Please read the respective "tips and notes" which explain that: https://www.duolingo.com/skill/de/The/tips-and-notes
Both "das Kind" and "der Junge" are nominative as both refer to the same person. There's no object.
Ofcourse it is. A definite child, the one which you know about (your nephew for example, when you are talking about him) is eating a boy. It doesn't matter which boy, someone from is kindergarten class for example, the point is that he's eating a boy.
If you use the ist instead of isst it becomes even more clear. The child you are talking about (again, your nephew for example) is a boy. A run of the mill, standard edition human boy. Like ~50% of other population of earth is or was once in their life.
I am a linguistic nerd. I think kind became kid, as child is essentially "high" English, if looked at the etymology late Old English 16th-17th "a youth of gentle birth" childe, also associated with french infant, or with child. Germanic languages show kiltham, Kilpei, kulder. Baby, said to first seen in writing 13 to 14c baban. I could see maybe French baba, to Scotts bairn.
Well, words seldom match 1:1. It is just as complex the other way round. If I find the English word "that" it needs to be translated by different German words: before a noun ("that child") it is "das", "jenes" or sometimes "dieses", standing in isolation ("that is funny") it is usually "das", sometimes "dies", as a relative pronoun ("the child that I know") it is "das/der/den/die/dem" or "welcher/welchen/welche/welches/welchem" deopending on gender, number and case, and as a conjunction ("I have heard that he is ill") it is "dass".
So you could say "that" is even more complex than "das". But I would not say so. The message is that some words are in fact more than one word but we don't recognize this in our own language before trying to learn another language.
1.) the feminine "the" is not "das", but "die". "das" is the neutral "the".
2.) grammatical gender has no relation whatsoever to natural gender. It is simply attached to any noun. It is a coincidence that "Kind" is grammatically neutral, it could have been masculine or feminine as well E.g. "das Mädchen" (= "the girl") is grammatically neutral.
Yes, the "role" of "the" does not change from language to language, It acts as a definite article to denote a familiar or mentioned subject. So using it for subjects that are not any of it is logically wrong. In this case you use "das" because the following noun (Kind) is Nominative,singular and neuter ! Be aware that "das" is also used as a pronoun meaning "this" or "that". I hope it helps !