Well, the sentence is pretty clear about the fact that there are multiply children (They use 함께, meaning ''Together''), but they also added 들 on to the word 아이, making it plural.
I think.... I have not been studying this language for long, just thought it made sense:)
Yes, 들 is necessary. In English, we pluralize nouns even though there are other words to indicate multiples in a sentence. Even within that sentence the verbs would indicate plurality but without 's' it wouldnt necessarily be clear what I'm talking about. The reason is to clarify the subject. What I mean is, if we only had 함께 it isn't clear that it's children who are sitting together. There could be other things sitting together. So the 들 clarifies that the subject of the sentence are children rather than a child and something else. Plus, I think the necessity of it would be clearer when sentences become more complexed. I'm not a native speaker, but this was how I understood it.
들 indicates plural form. Similar to 's' in English. However, I consulted a Korean friend about this, and she told me that while it is mostly necessary, it can often be left out since plural form can be indicated through context rather than saying it directly. This isn't the case in English, so it can be hard to understand. If you don't speak Korean well, you can just say 들 every time you want to pluralise something, however, if you did this while speaking to someone who is fluent, they would probably think you're a bit strange.
it's 'an'. you can check assimilations here https://www.duolingo.com/skill/ko/Alphabet-2/tips-and-notes
Is there a big difference about it being "The children sit together" because I wrote "The children are sitting together" And on other questions I aalways write like "laughing" when it's supposed to be "laugh" But it is basically the same so is it wrong to think about it this way?
If you are asking when to use 이 or 가/은 or 는, 이 and 은 are used when the word's last character is a consonant, and vice versa. If you asking what they translate into, they translate into nothing and are simply used to identify which word is a subject (i.e, "I" in "I eat rice"). If you know all of that and are confused about when to use 이 or 은/가 or 는, I do not know.