There are just as many options in Children's book as there are in fiction novels. The most common would look like the attached picture. Images should give enough space .125 inches, though I suggest .25 to be on the safe side. Make sure your illustrator knows the size of the book and the extra space needed for proper printing.
The words of the story should stand out from the pictures. The options come in at what age range the book is for. The below is for very young readers. This is why the text is large and colorful though easy to read. The older the child the smaller the text is a good rule of thumb, not to go below 12, though I'd suggest 14 point font up to adult novels.
Other than board books the standard minimal page count would be 24 and for first readers I don't suggest much more. The page count can go up from there.