That's what I thought, I filed a mistake with duolingo because they didn't accept kids as an answer in place of children. Can you guys also file a mistake
It's not מאני because only the object pronouns attach to prepositions like ב or מ while אני is a subject pronoun. As for why it's not מני, honestly I don't know lol. But to the best of my knowledge it's just one of those things in language where there's no rhyme or reason to it and you just have to memorize it.
Probably, מ- just used to be מם or מן once and here it is still present. The same function in Arabic has the preposition is من (min), so I guess their version is closer to the original.
That's correct, in fact there are certain cases in which מן by itself is still used (I'm not sure what the grammatical rule for it is, though).
For example, there's a Jewish blessing that goes "ברוך אתה ה' אלוהינו מלך העולם המוציא לחם מן הארץ" - blessed are you, Lord our God, King of the Universe, the one who brings out bread from the earth.