1那孩子 that kid/child
This can be used in reference to “the” only one child. The one that you particularly refers to.
2那些孩子 those kids/children
This can be used in reference to “those” children that you particularly refer to. 些 is always needed to specify plurality. So if you refer to children, then you have to add 些。
3那些孩子们 those kids/children
This one is same as above.
Likewise, 些 specifies plurality, so does 们。孩子 could be used in reference to single or plural, but when you add 们，孩子们=children，is plural。Then you can’t say 那孩子们，because 那could not specifies plurality. So when you say 孩子们, 些is nessesary.
For your question, you could use both 2 and 3.
When you say those kids or children, you don’t have to know them, but you should knowledge those kids that you refer to are a certain group of kids, like, those kids in white shirts, those kids down stairs, those kids are painting, etc. In that case, 2 and 3 refers the same meaning.