Here are some answers:
1. If this is nonfiction, photo illustration make sense, but if this is a informational fiction, it may well be illustrated. The difference is that informational fiction has a fictional element. For example, the tooth speaks. If the tooth is narrating the story, then maybe it should be illustrated. Some works contain photos that are manipulated to give a sense of the world and characters. Your story sounds like informational fiction.
2. It can be better to think in terms of two-page spreads instead of unique pages. You need 12 or 16 spreads. Some may contain two images, one per page, but others will have a single image or even a few smaller images. With a picture book, art is carrying as much story as text. Look at bunch of informational fiction and see how they are laid out if you can. A children's librarian is a great resource for things like this. Look at nonfiction too to see which your book fits with.
3. Children's books is a broad term. Some people use it to refer to picture books and up. Others may only want chapter books and up. Other categories are middle grade and young adult novels as well as graphic novels in both fiction and nonfiction. If an agent says they take children's books, look for images or descriptions of the books they rep. Sometimes you'll see picture books and sometimes you won't. Also see if the guidelines mention subbing a picture book. And look for interviews and MSWL (manuscript wishlist) info to see what is mentioned there. In other words, Query Tracker is just the starting point. An agent website is the most accurate.
I hope this helps. Good luck.