Notice that Kinder is plural. Consider that in English you could say "Not all children like apples" but you could not say "Not every children like apples". To use every you would need to change the sentence to "Not every child likes apples". It's similar in German.
Nicht alle Kinder mögen Äpfel
Nicht jedes Kind mag Äpfel
The ending matches whether you would otherwise use der/die/das/etc.
das Kind --> jedes Kind
der Mann --> jeder Mann
Case might also affect it, e.g. Ich gebe jedem Mann einen Apfel ("I give every man an apple"), because it's dative case (it would otherwise be dem Mann because der becomes dem in dative case).
Why can't it be "Alle Kinder mögen Äpfel nicht"?
Is my understand correct as below?
"Nicht alle Kinder mögen Äpfel." - Not all children like apples. (The negation is on the subject)
"Alle Kinder mögen Äpfel nicht." - All children does not like apples. (The negation is on the whole sentance.)
"Alle Kinder mögen keine Äpfel." - All children like no apples (or) All children does not like any apples. (Negation on the noun.)
1) "Nicht alle Kinder mögen Äpfel." - Not all children like apples. (The negation is on the subject)
So there is some child who doesn't likes apples.
2) "Alle Kinder mögen Äpfel nicht." - All children do not like apples. (The negation is on the verb / the whole sentence.)
So every single child does not like apples; there does not exist a child who does like apples.
3) "Alle Kinder mögen keine Äpfel." - All children like no apples. (The negation is on the object)
Similar meaning to the one just before. Every single child does not like apples; there does not exist a child who does like apples.
It's clarification but not confident as I red it some where else. Because structure of German sentence is baffling me except one.
verb always comes second.
In compound sentence if the main clause comes first then verb comes second and the verb of the dependent clause comes last (doubtfully). If the dependent clause comes first may be verb comes second (doubtfully) and the verb of the main clause comes first after comma.
Let me not divert the above query for nicht.
In German sentence structure, the main idea of the sentence comes first, then the verb (modals push other non-modal-verbs to the end of the sentence and change their inflection to infinitive, btw).
The thing before the verb is often the subject, but can be an idea- whatever you want to place emphasis on.
This sentence could have also been "Äpfel mag alle Kinder nicht", if you wanted to place the emphasis on the apple part, as in "not all children like APPLES". Whereas the sentence in the exercise placed the emphasis on the 'not all children' part, so it is the main idea and so, comes before the verb.
More on sentence structure here:
all and every means the same thing
Not quite -- "all" is used with a plural noun (e.g. "all boys") and focuses on the group as a whole, "every" is used with a singular noun (e.g. "every boy") and focuses on the individual members of the group.
What was your entire answer?
The pronunciation is different.
By the way, it's totally worth reading the entire guide.