Why would it be a' chlann rather than 'na chlann', being plural, pls? :)
I think because "clann" on its own already means the plural "children", not "child". So even though it represents a plural amount, it's (grammatically-speaking) a singular noun.
Exactly. Incidentally if you for some reason really want to make use of "clann" for the idea of a single child (as opposed to other words like "pàiste" or "leanabh") you can say "duine-cloinne".
I would be interested to know how common that word, that I have only seen in a dictionary, is.
Here is a link to a page with a lot more on the subject (mostly by me).