I have to admit I’m not clear on how kids learning hiragana first reflects on the use of kanji – kids learn hiragana first because without it, there’s no context for the kanji. Basically, hiragana have inherent sounds, but no inherent meaning. Kanji have inherent meaning, but (for the most part) have multiple readings.
The English equivalent is that hiragana are like letters and kanji are like words – you teach the letters before you teach the words, because otherwise there’s no way to explain how words work. Trying to write Japanese without using kanji is like trying to write English without using words – just handfuls of letters that happen to make the same sounds. To someone listening to them spoken, it’ll probably make sense, but someone reading it would struggle to make heads or tails of it.
And as analogies go, that one’s fairly terrible, but it’ll do…