A child's world is a unique place, full of intrigue and mystery, and it is natural for this to be reflected in the bedroom. When decorating a child's room, bearing this in mind is one of the keys to creating a great room.
If you choose to theme the room, bear in mind what the child likes. If they enjoy football, perhaps a football theme room would be ideal. Or perhaps a fairy tale theme may spur your child's imagination. However, ensure your child will be happy with the design - what a child may like one week can quickly change into dislike, perhaps due to peer pressure or TV. Ensure you choose the one of child's long-term interests, rather than a passing fad which may embarrass them as they grow older. A more generic theme, such as football or sport, outer space, dolls or cars is better than a specific theme (for example, a football team or a cartoon character), as it can be adapted much more easily.
Of course, there are other considerations to take into account when decorating and designing a child's room. Furniture should be sturdy, as children love to jump on their beds. The floor should ideally be wipe-clean, as accidents will happen (especially if the floor is carpeted!). Also, you may wish to use wipe-clean paint for the walls - as your budding artist will not want to be constrained by easel and paper all the time!
Typically, boys rooms are themed around sports, cars and outer space. Many are decorated with superhero designs as well, which can be a particularly delightful scheme for a child. Artwork from the child's favourite cartoon can also be used - it all depends on imagination! 
However, you may wish to paint the room simply, so that it will not need to be updated once your child decides not to like a certain character or TV programme. In this case, you will need to select colours. Blue is the 'official' boys colour, although there are a number of others which can be used without being seen as 'girly'.
For a girls room, the most popular theme is fairytale, though a number of rooms today have angels, either the traditional, or a modern stylised version. If no specific theme is wanted, pinks and pastel colours should go down well, maybe adding a few graphics such as flowers or butterflies, which can add a 'secret garden' hint to the room, without completely theming it.
In many cases, it is the furniture which can theme a room as well. A girl may want a more intricately designed bed, whereas a boy may prefer to have a bunk bed with space beneath for a den or a comfy seat. It is not just colour that can define the gender of a room, though the colours blue and pink are intrinsically linked to boys and girls respectively.
Of course, your child should be involved with the room as well - ask what their likes and dislikes are, and get them involved with the process - it is their room, after all!