As a parent of a 7 and 5 yearold, the house is pretty full of imagery for children from birth to age 10. I am constantly looking at what is on offer. There is so much on offer that it is really impossible to put anything sensible on paper. I am not sure that there are really any hard and fast rules about illustrations for different ages, but the way in which one communicates an emotion or a situation might change... or it might not... An example of an illustrator who appeals to ages from 0-100 is Quentin Blake - and he does this without really changing his style of drawing, just by adding detail.
But to go back to the beginning..
For Baby and Pre Readers, there are developmental stages to observe... what colours and shapes they can recognise through to understanding the way that words and sentences are formed. Initially my children has black and white books as that is the way we first see the world. After that we had to have cloth books that they could suck which had complementary colours to stimulate them... but then the fun stuff started...
Some of the best early books are tactile - featuring different textures for sensory stimulation. One of the best examples is
the Touchy Feely series published by Usborne and illustrated by Rachel Wells. They are packed with different shiny scratchy and slimy things to touch and a mouse to spot.. (6months-18months but my kids came back to them when they started reading)
Then there are books that develop language skills e.g. Opposites
Maisy's book of Opposites - is a good example of this... by Lucy Cousins (ages 1-3)
Up to this point illustrations are the main thing and the words are minimal. From there we begin to get stories developing - my children loved the Mick Inkpen stories (Kipper and the Lullabyhullabaloo - simple appealing characters with expressive faces - and very funny prob 2-6ish). The stories and illustrations are often best done by the same person as they need to work seemlessly. Helen Oxenbury's classic 'Bear Hunt' stimulates the imagination with simple drawings. Interestingly they did bring out a pull out book of 'Bear Hunt' but my children were so obsessed with pulling the bits that they missed a lot of the story itself, so I hid it away and we found we enjoyed the plain book much better.
Children's books develop the idea of sentences and reading from left to right (in English) The Duck books are bright and cheerful by Jez Alborough (2-5) and the children used to look for picture clues to predict the mishaps that would befall the Self important Duck character.
They are bright, cheerful, characterisation is uncomplicated and humourous and they are great for introducing the idea of reading....
They also loved the combination of photo montage and illustration in the Lauren Child books.
Axel Sheffler's illustrations always have wonderful details and simple humour in them, again the animals are never too scary e.g. Gruffalo is funny not scary even though it is clearly scary in the context of the story. (2-6) - How to deal with 'Scary' and 'Sad' stuff in stories is a very interesting problem for the illustrator. You have to hint at scariness without giving the child nightmares.
The Sir Charlie Stinky Socks books by Kristina Stevenson do 'scary monsters' really well and they combine text and image very succesfully, with a playful use of fonts and pull out flaps etc. (ages3-7)
As children start to read they really enjoy books that give them more details in both the story and the images to get stuck into... Winnie the Witch has fantasticly grotesque gothic images, but the boys love them. (4-8)
As children develop they like to role play adventures and the 'Dragonology' books are great for this. They are packed with maps and pull out flaps, letters, documents, fake facts all presented as if they are a historical scientific record (ages 6-12ish). The illustrators have taken inspiration from old maps and documents and they are presented as if 'real'.
Then there is the presentation of History - Horrible History style. This series has hit the mark big time. The Cartoon illustrations compliment the presentation of the historical facts... it's a grotesque and funny representation of some pretty ghastly moments (6-16)
Oliver Jeffers 'Incredble Book Eating Boy" is one of my favourites. I love the use of pages from old books and different texts along with the simple sketches. I first read it to the boys when they were 2 and 4 and they have recently re-read it and they commented on the illustrations which they think are fantastic... there is something 'naughty' about tearing out sheets from a book and painting over them and this is part of what appeals to them.
Horrid Henry is a particular favourite.. there is a picture book version for younger children, with more colour and less 'horribleness'
Tony Ross's simple prints are wonderful characterisations and Kevin McAleenan has clearly just made Henry a little chubbier, less 'evil' looking and more approachable for the younger audience - gently leading the reader towards the original Horrid Henry books which my children have picked many bad habits up from.
The illustrations in the established reader e.g the Beastquest illustrations are quite scary for younger readers as they are quite ferocious and fiery and adult in their use of 'fantasy', inside they are quite traditional almost like old fashioned etchings that is in keeping with there style. The images become less important to the text at this point - they are more 'dressing'
On the other end of the scale - McKean's illustrations in 'Varjak Paw'
and 'the Savage' are also very 'adult' they do not condescend to their readers, but they add so much to the stories... it is impossible to imagine the books without his illustrations.
There are also Graphic Novels and 'comic strips' available for this age group... and also for established and developing readers. This is an interesting area...
As children develop - they are able to appreciate more complexity in both text and image, and both visual and cognitive imaginations can be increasingly stimulated - and so illustrators often employ more subtlety and suggestion within the imagery according to the age of the target reader.
I think that this developing ability is really the main difference between younger and older readers, but that is not to say that the simple and direct has to be purely used on the younger ages, it can be extremely powerful across the board.