Before you start creating your children's book for Amazon KDP or IngramSpark, you'll need to choose your trim size. Your trim size is the actual final size of the book when you hold it in your hands. (There's also the bleed size, a whole other subject, which I won't get into here!)
You need to choose your trim size first because it is very difficult to change later. If you're illustrating your book or working with an artist, the illustrations will need to be made at the proper size for the book. If the book size is changed later, they may need to be completely redrawn if they are too small. Also, it's going to be a lot of work to change your layout later if you choose a different trim size.
The best way to choose your children's book size is to visit a library or bookstore and browse the children's book section. Hold the books in your hand. Flip through them. See how the illustrations and text fit within the size and orientation of the book.
Some popular trim sizes for children's books are 6x9", 8x10", and 8.5x11" for vertical format and 8.5x8.5" for square format. If you'd like a landscape format, then Amazon KDP does offer 8.25x6". You can also enter a custom trim size into Amazon KDP as well, as long as the width is between 4"-8.5" and the height is between 6"-11.69". IngramSpark allows any trim size as long as the width is between 4-8.5" and the height is between 6-11".
Keep in mind that if you want to do a hardcover book, the process gets much more complicated. Amazon KDP requires a minimum page count of 75 pages for a hardcover book. I recommend starting with a paperback for your first book.