When my nieces moved to a new house, I wanted to give them a room that could grow with them from little girl to preteen. The older girl wanted a jungle room and the younger wanted a princess theme, so I compromised with an enchanted forest with a faerie tale castle in the clouds and jungle animals lounging in the "trees." I used muted colors to avoid the cartoon-y look children's murals can get, and used abstracted shapes instead of fully realized forms. You can see the entire transformation from 1980s wallpapered duck room to its current state here: http://hadesarrow.com/blog/2011/03/14/nice-niece-bedroom
The wishes of my nieces and a whole stack of kid's room magazines.
I used all Home Depot Behr paint (http://www.behr.com/Behr if you haven't used their color tool, I recommend it). Blue masking tape for the "trees" - in a few spots I used the special "painters tape" which I bought by accident. It does work better, but it costs three time as much as masking tape, which really annoys me since all it's doing is the job masking tape SHOULD do in the first place.
Probably the Koala. N1 likes the cat best, N2 of course likes her castle, but wants to know why I didn't paint the whole room bright pink.
Keep it simple. Use a limited color pallet, and if you find yourself trying to figure out how to tackle a large, complicated aspect of your design, scale back.