So yes, the question is valid
I'm not saying the question is not valid, and of course a child's question is perfectly reasonable because they (usually) want to learn and understand.
I was more perplexed by the supposed answer from the mother which, in JW terms, is completely "wrong". It's not even slightly off, as perhaps being a bit confused or missing some detail - like you say, a lot of JWs can't properly explain their own beliefs, especially in recent years. But no, it's the complete opposite of what they teach about the soul and death - as any glance at pretty much any "bible study" book they've produced in the last 50 years or more would show. It's basic "JW 101", as Americans might say - along with "what's God's name" or "who wrote the Bible".
You yourself almost kind of summed up the JW belief in the last paragraph in simple terms that could be used to explain to a child, when you said "God converts their [...]spirit into something that then goes to heaven". I would perhaps reword that as "God recreates/re-forms the person in the heavens as a spirit - with their memories, personality, etc" - to make it clearer from the JW theology perspective that strictly speaking they don't believe anything "goes up to" heaven - that humans don't have any eternal floating part of them that drifts off, and they don't take their physical body with them.
According to JW theology, what "returns to God" is the ability for a man to live again, the "breath" that He gives a human at the start of their life. In the Insight volume on Soul, it's explained as:
Eccl. 12:7: “Then the dust returns to the earth just as it happened to be and the spirit [or, life-force; Hebrew, ruʹach] itself returns to the true God who gave it.” [...] The text does not mean that at death the spirit travels all the way to the personal presence of God; rather, any prospect for the person to live again rests with God. In similar usage, we may say that, if required payments are not made by the buyer of a piece of property, the property “returns” to its owner.) (KJ, AS, RS, NE, and Dy all here render ruʹach as “spirit.” NAB reads “life breath.”)
That's also linked with Psalm 146:4 "His breath goeth forth, he returneth to his earth; in that very day his thoughts perish." (KJV) - so there is no thinking, no consciousness, that lives on separately. Only if God recreates the man - as either a spirit being in heaven or back as a material fleshly being on earth - will he live and have consciousness again.
Any JW who was even half paying attention to what they were studying should be able to answer that in brief at least, along the lines of the sentence you said, even if they can't go into detail about nephesh or ruach or whatever (which would not be necessary or meaningful to a young child anyway). But for a JW to say "a soul was inserted" that could then "go up to" heaven I just thought was... very odd.
Apologies, I'm not intending to derail this topic, but Vanderhoven did ask me to elaborate and I thought Anony Mous's comments were also worth following up on.