orthography - Spaces in children's books

Written Japanese ordinarily doesn't use any spaces to separate words. But Japanese children's books do.

For example, the opening of Tomi Ungerer's The Three Robbers (すてきな 三にんぐみ)

あらわれでたのは、

くろマントに、 くろい　ぼうしの　さんにんぐみ。

それはそれは　こわーい、 どろぼうさまの　おでかけだ。

Are there general rules by which spaces would be inserted? (For example, always after particles, always at "word boundaries" (suitably defined).) Or do editors/authors just insert spaces at "natural" reading pauses? (Natural for small children at least.)

For example, in the same book きがついた is written without spaces (which would be fine if the rule were "no spaces in phrases which are a unit"), but then だれも かれも is written with space, but I think of the phrase as a unit.

Answer

There's a quick way to know this. The place where a space can be inserted is roughly the same place where ね can be naturally inserted.

あらわれでたのはね、
くろマントにね、くろいね、ぼうしのね、さんにんぐみ。
それはそれはね、こわーいね、どろぼうさまのね、おでかけだ。

Actually this structure is known as 文節. Basically, a 文節 starts with a noun/adjective/verb/adverb/etc, optionally followed by one or more subsidiary verbs and particles.

Inserting spaces between all words (i.e. あらわれ で た の は) is overkill in most cases.

But this rule is not strict, and you will find a lot of exceptions in children's books and old video games. Don't worry too much.