Could this also be "a" child or is "the" the only correct answer?
Both are correct.
Is it possible to translate "dziecko" as "kid"?
Yes it is.
Why is "The kid eats a bread" not correct?
"Bread" is an uncountable noun, so "a bread" (meaning one bread) is grammatically incorrect.
I've just lost my 1 heart...
Why is "Child is eating bread" incorrect..?
Cuz before this sentence i used to translate in present continous...
Your sentence isn't correct in English, because you need to use an article before a countable noun like "child".
Why is "the" child, why isn't "a" child?
Both are correct and accepted.