We had a wonderful picture book version of the second coming and some of the Revelation issues for young children. It was called, “Jesus is Coming Again”. I have to qualify by saying: a) Not really Charlotte Mason-y and b) Somewhat cartoonish in the illustrations (I prefer realistic illustrations). BUT, as you mentioned it is very difficult to find books about this geared for children! You can find it used on Amazon for about $2. Here is a review on it from Amazon:
“This is the only book I know of that tells about the rapture of the church, the 2nd coming of Jesus and living for eternity with God. The book takes a somewhat complex subject and simplifies it truthfully. It explains so clearly the future of the Christian. There are beautiful, happy illustrations on each page. The judgement is mentioned briefly. A full page of “scripture notes for parents” give the Bible verses to verify each section. There is another page of teaching tips for parents and a picture to color at the end of the book. This book really helps to understand what is ahead for the believer. It is written on a child’s level but I believe it is helpful for adults also.”