I would 100% assume he heard this in school.
Trump is the most hated (polarizing) president of all time, and there isn't a close second.
With how much trash people are talking about Musk as well I could see a child deflecting away from the parent and onto the "source of the problem".
I'm pretty sure Elon's kid is homeschool, but your point remains intact. He must be hearing all sorts of crazy negative stuff around him all the time.
That's funny I was going to say "unless he's homeschooled or something..." but then I left it out because yeah there's some serious Musk derangement syndrome going on ever since he bought Twitter. That's a double whammy for that kid.