But I do have a handful of simple guidelines concerning page composition that serve me well---and that you'll find are pretty much universal to any successful picture book.
1. Set margins for the page. I usually set my page margins at around 3/4" to 1" for the top and front edges; around 1 1/8" for the gutter and the bottom edges. This allows enough room for thumbs and fingers to hold the page without covering up anything of importance. And these margins dimensions I give (and that show in the photo above) will appear in the finished book to be roughly the same. The curve of the pages turning into the gutter means that the wider margin actually looks the same as the top and front margin, optically. And the bottom margin, just because it is at the bottom, appears to be the same as the front and top---again, an optical illusion.
2. Keep anything of importance within the margins; and if it's of great importance (e.g. the main character), keep it well within the margins. This doesn't mean there must be frames or borders for the art. Even if the art is going to bleed off all four edges (i.e. run off the edge of all four pages), a margin will help keep important elements where they will have the most impact. Think of a theater stage: if a character or element is too close to the wing, it gets conflated, visually, with the edge of the stage. It loses the audience's focus.
3. The center of the page, as indicated above in the photo, is the place of stasis on the page. Keep important elements off-center. Even if the overall composition is intended to be more static, place the main character, for example, off-center. (Clearly, I believe that movement is critical to a successful composition.)
4. General movement on the page must always be from left to right. We read (in English) from left to right, and turn book pages from left to right. In a book, we expect the "next thing" to be to the right, not to the left. Since children are learning how books work when they use picture books (even if they can't actually read yet), it is doubly important to be consistent in this.
5. When composing a double-page spread, don't let anything of importance (such as a character) run across the gutter.
There are of course many other considerations at play when I am composing pages. These five are my basics.
My usual caveat: There will be successful exceptions to everything I've said here. And now, having stuck my neck out to a dangerous extent, I will quickly withdraw it before any rotten tomatoes find their mark!