So does adding 는 make it passive then? A walking child vs the child walks
는 turns a verb into a present participle. Passive would be "the child is walked".
I'd agree with mas925, "walking" describes the baby, kind of like in the sentence "the walking baby smiles". "Smiles" is the verb there and "walking" simply describes the noun.
When I hovered over 걷는 it says "who walks", but "the baby who walks" isn't accepted. It means the same thing as "walking baby"
It's gi ㄱ has the pronunciation of g or k( I see you're Indian so maybe you'll understand ग or क) while ㅋ has the kh(ख) pronunciation. To anyone who doesn't understand, ㅋ/ㅌ/ㅊ/ㅍ are pronounced with air from your mouth
The authorized translation here ("walking baby, not baby who walks") is ignorant of the fact that "walking baby" and "baby who walks" are parallel adjectivals in English. Even Duolingo's own "grammar" lesson points this out and says the adjective and relative clause can be parallel. The "teacher" needs to learn some rudimentary linguistics..