Book which is read by the parents with the kids beside, needs a lot of pictures/illustrations as the kid will be interested only and only in the illustrations. A child will relate the entire story looking at the illustrations only, so it is very important to add a lot of illustrations to the small children story books. A child, who is learning to read, also shows interest if she/he finds very small interesting words. So adding the illustrations and relating them with small words attract the children around 5 to 7 year old. Children 10 and above can do with the less illustrations, but importance of illustrations will never be reduced, as visuals can always impress the human eye of any age.
Various styles of illustrations can be seen in the market, but good presentation of illustrations in a book format is also equally important.