The Silmarillion is more child-friendly that one might think
TL;DR: I read the Silmarillion to my 6-year-old son and he loved it.
My son loves fantasy and monsters. We have some Lord of the Rings Lego sets, and he had seen several glimpses of Tolkien's universe, including some short videos on YouTube. He was especially fascinated by the character of Melkor. (I probably influenced him on that point; I am a big Melkor fan myself.)
Some months ago (when he was 5) I figured I would try to read The Hobbit to him. The first chapter was a slog to get through, and he complained that it was slow and boring. But after that point he got really into it. We would read a bit every evening, and often he would ask to read a bit extra during the day.
We watched the movie trilogy concurrently. After each 1/3 of the book we would watch the movie that covers the part of the book. (I am one of those people who LIKE the Hobbit movies.)
He loved The Hobbit. One detail that particularly fascinated him was the lost realm of Gondolin, where the swords Glamdring and Orcrist were forged. He kept bringing that up and wanted to know more about it. I had also told him about Fëanor and his creations (the Silmarils and the Palantíri), and he loved hearing about that.
So I figured, let's try to read the Silmarillion.
And it went great! It was slow going, because it was sometimes difficult to understand, and he would often have questions and need explanations or reminders. Sometimes we would only get through 1-2 pages per evening. It took several months to get through the entire book. His interest waxed and waned; some nights he preferred to pause the Silmarillion and read something easier. But he never dropped it. He was especially looking forward to the parts about Gondolin and about Eärendil.
Tolkien's prose is difficult, so I would sometimes paraphrase in simpler words instead of reading aloud. (Our native language is Danish, so I read it to him in Danish translation, but the translated prose is also difficult.) I would also sometimes skip boring paragraphs. There is a vast number of character and place names to keep track of, so we often had to pause and look at the map or revisit who is who. I often reassured him: "I can only keep track because I have read it several times before."
We also read the Akallabêth and The Rings of Power and the Third Age. We have the Lego Barad-Dûr, and he loves that, so he was excited about the story of Sauron and the One Ring.
For children, one great thing about the Silmarillion is that the pace is relatively fast, since it is structured more like a summary of events than a novel. I want to read The Lord of the Rings too, but I am not sure whether he is ready for that, since the pace is so much slower.
In summary, though, reading the Silmarillion for my 6-year-old was a smashing success. Would recommend. :)