It's a grammatical rule in the same bucket as "i before e except after c" and "possessive apostrophes on words ending with s don't get another s" "i before e"

It's a grammatical rule in the same bucket as "i before e except after c" and "possessive apostrophes on words ending with s don't get another s" "i before e" apparently has more words that violate the rule than follow it, and the possessive apostrophe rule runs opposite to spoken English: "This is James's book". The fewer/less thing is a general rule, not an ironclad one - there's plenty of examples where it sounds natural to use either. I'd also argue that with a demographic the size of Australia (~22M), the number of children in it are not feasibly countable (like grains of sand), and as such, 'less' is fine. :)