I don't understand. When do I use ett and when do i use en? After all, don't they mean the same thing?
They're different genders, just like e.g. un and una in Spanish, but with no biological connotation.
For some more info, please have a look here: https://forum.duolingo.com/comment/26420394/Answers-to-some-common-questions-on-grammar-that-beginners-have
If 'ett' in this sentence is 'one' how would a person then say 'the' to make the sentence read 'the child....'?
I don't know why they expect us to get these hearing questions right. They are very, very hard to understand especially if they don't sound the exact same as we say it.
Try pronouncing it like "myolk" and then replace the "o" sound with how you would pronounce the "u" in "burn"
You don't really, it's mostly context. (in speech you'll hear it because it'll be more stressed when it means one).
Is the 'is' here really a mandatory translation? Why is 'A child drinking milk' inaccurate?
Yes. 'A child drinking milk' would be ett barn som dricker mjölk in Swedish.