Diversity is all about accepting that we are all different but all equal - celebrating our differences and similarities.
With young children, you could try to initiate a natural discussion on differences by looking at pictures (or make them into jigsaw type puzzles after laminating them?) of children in wheelchairs, children in different ethnic dress, somebody with a guide dog...
- just answer any arising questions very honestly.
Looking at similarites and differences could also be done with artwork - getting the children to paint self portraits, draw pictures of their families etc - what bits are the same (who has blue eyes, a sister, long hair...) and what is different (who is an only child, has their hair in bunches...)? One of my colleagues asked the children to draw round their hands :talktothe and then decorate the hand shapes - which lead to a lot of discussion on differences and similarities.
Ensuring that your setting has resources and pictures which show diversity is important - too many resources show blond haired, blue-eyed, fair skinned children without disability. Make sure that different races and disabilities are also represented.