"this, that, these, those" can refer to people as well.
For example, you might introduce someone as "This is my sister."
Saying "She is my sister" would not make sense, because personal pronouns generally refer back to something that had been mentioned before, but if you're introducing her, you haven't spoken about her before.
Similarly here. You are pointing at a group of children that you hadn't spoken about before, so you have to use "those" rather than "they".
No. That would mean "These ones", i.e. "from out of the group of [something] that we had been discussing, these [somethings] are girls". There would be some noun implied. For example, "There are eight children in the room. These ones (= These children) are girls."
But if you're introducing something new to the conversation by (verbally) pointing to it -- "These are children." -- then you use neuter singular to do so in German: das or dies. Regardless of the gender of the new item(s) or how many there are.
Maybe it's just me, but I can't imagine a situation in which I would say "These are girls". It sounds really weird to me. I would always translate "Das sind Maedchen" as "They are girls". "Those are girls" works, but "These are girls" just sounds strange. It's almost like it objectifies and dehumanizes the girls.
One context I came up with: you're showing someone a photograph taken from far above (perhaps from a quad-copter drone) -- so you can't make out anybody specific.
You're showing your friend the picture and pointing at some blobs visible on the picture. "See these blobs? These are girls and those over here are boys."
"They are girls" would not work -- at least for me -- because "they" refers back to something already mentioned, but you hadn't talked about people yet at all; you're just starting to talk about them by pointing to them.
Because the German has Das sind Mädchen (= Those are girls / These are girls) and not Sie sind Mädchen (= They are girls).
"this/that/these/those" refers to something new that you are "pointing at" and introducing to the conversation; "they" refers back to something you had mentioned earlier in the conversation.
When you introduce something new to the conversation with "this/that/these/those" (not followed by a noun), that is das or dies in German - always neuter singular, regardless of what kind of thing or how many of it you are introducing.
If, on the other hand, the word is before a noun, you have to use the appropriate gender, e.g. das Mädchen for "that girl" and die Mädchen for "those girls".
First off, Madchen should be spelled Mädchen, or Maedchen if you can't make the ä letter.
Next, sind is the third person plural verb, which means that Das sind Mädchen can only mean "Those are girls."
"This is a girl" would be Das ist ein Mädchen. -- using not only the third person singular verb ist but also the indefinite article ein. (Saying just Das ist Mädchen would be like saying "This is girl".)
When you're introducing something new to a conversation, you always use neuter singular to do so (dies or das), regardless of the gender or how many things there are. Even masculine nouns such as Hund or Mann or feminine nouns such as Katze or Frau would have Das ist ein Hund / ein Mann / eine Katze / eine Frau.
But the verb that comes next takes the number (singular or plural) of the thing(s) you are introducing -- sind for plural, ist for singular.
just curious about this: why can't 'seid' replace the word 'sind' in this sentence?
Because the German doesn't say Sie sind Mädchen (They are girls) but instead says Das sind Mädchen (Those are girls).
It doesn't talk about some people you had previously been talking about (they) but introduces someone new to the conversation by "pointing" at them with the word "those".