> I've tried reading some Japanese-language books meant for kids around age 10...

I’m not sure at what target age kids’ books stop using word spacing, but books for younger children generally use it. Nevertheless, if you are used to seeing words written in kanji, even with word spacing an all-hiragana text can still trip you up, for the reasons you noted.

Side comment: Something I haven’t seen remarked on much is how Japanese can be easier for small children to start reading than English is because of the nearly one-to-one correspondence between character and sound for kana. My two daughters and now my six-year-old grandson have all grown up with Japanese as their first language, and they all started reading hiragana-only children’s books earlier and more easily than I, at least, learned to read English when I was a child. My grandson has also picked up katakana on his own; he is into dinosaurs and his picture books give the names of dinosaurs in katakana.