AFAIK numbers and counters are usually used in four ways which are following:

-X, number, counter (아이 두 명)

-num, cntr의, X (두 명의 아이)

-num, X (두 이아)

-X, num (아이 둘) (notice the 둘 instead of 두)

They can be used rather interchangeably, however, the first one seems to be more popular than the rest. Also the second one is supposedly used mainly in writing.

In North Korean orthography, there is no space in front of the counter. Therefore: 아이 두명 and 두명의 아이.

I think you could add one somewhere but it is not necessary.

두 is the number

두 아이의 사이 means "between 2 children". 사이 means gap between something. 두 아이 is 2 children. 의 is "of". So, it literally means gap of 2 children.

사이 means like "middle" so 의 here means the middle that belongs to the children, so "in the middle of the children"

It indicates posession. In this case, the space (사이) "owned" by the two children. I guess a literal translation would be "between the two children's space".

Right -> your right

Left -> your left

Between -> your "between". Of course in English we don't say "your between". :-)

What Duolingo (and most materials designed for beginners, for that matter) calls prepositions are actually nouns. For example, think of 아이의 옆 as "in front OF the children." The logic works for all Korean "prepositions," though it doesn't really hold for English. Does that make sense to you?

Same question as this guy.

It isn't necessary, as the number if children is expressed, and the plural marker 들 is more often used for emphasis that there really were a lot of something.

Even though 들 indicates plural, it's not always mandatory to use

The absence of 들 does not mean singular.

It is already plural because of 두 being placed before 아이의

I dont think so since it already specifies that there are two children.

I would say it like this, "두 장난감이 아이들의 사이에 있다"... Not sure if it is entirely correct though

the 이 in 장난감이 is a subject marker, so 'toy' is the subject of the sentence. In your sentence, 'There is' acts as the subject.

I think this is one of the times when word order does matter. If the sentence was "두 아이의 사이에 장난감이 있습니다" that would mean "There is a toy between two children."

That's actually not correct english.

Baby is 아기, child is 아이

아이남자 and 아이여자 have the same meaning as 아이, but just specifies the gender. Baby is 아이