Ask your child!
What would he/she like?
Don't forget this is your child's' wall mural, not yours.
You might think a princess mural with fairies and flowers all cute and pink is the way to go, but she really wants a football mural.
Be careful with younger children. Get the idea of what they want and all their imput at the beginning but don't let them continue to add ideas on a whim once the project is under way. (I say this from bitter experience.)
With my own children I have made sure that what they want is not just a passing whim that will be forgotten a week later. Talk and listen to them over some time to see what subject keeps coming up. Take your time, don't rush into a decision unless you intend to change the room frequently.
Make a list of their interests and hobbies. The key is to eliminate those which you know to be just the latest fashion. Fashion changes quickly, sometimes overnight. Hobbies and interests are likely to stay with your child throughout their lives.

You may already have a good idea of what you child would like in his or her room.
You can now set your Mural Budget
Step 3

If not and you need some inspiration
Click here for kid's mural themes?