I tried just reaching hands
But liked a few faces to pile the pressure on
which when coloured gives you
I think the message of this image is more subtle than that required for a pre-school child but children who are not reading take a lot more information from the pictures and are visually quite sophisticated so I don't think it is necessary to "spell out" what is happening in each image. Pre school children are not aware of what they are supposed to take from a picture so it is important not to leave too much distracting information in a picture as it could lead them away from the flow of the story though having lots to look at and discuss can make a book more interesting especially when you're reading it for the 100th time!
A variety of materials are suitable for childrens illustration and I don't think that bright colours are necessary if the picture is visually attractive and the narrative is interesting. I don't think that most children are prepared to work at getting in to a story and I think they prefer images which are accessable and relate to their world. The examples I researched used a diverse range of materials and methods.
Age ranges are very fluid especially as modern authors work to make their stories attractive to adults who choose and often read the books with their children. Children mature at different rates so in a random group of the same age some will be reading independently, some not. Those with older or younger brothers and sisters will be exposed to different material which will affect what sort of story they are ready for or interested in.
I don't think that there are clear rules for what will work for children beyond the obvious ones that they should not contain adult themes though traditional fairy tales are quite violent and Judith Kerr killed Mog at the end of her series of books, maybe sex is the only taboo?