I think one is to know how to be playful, making normal procedures and actions a game, from peek-a-boo to turning an asthma spacer into a magic space mask. Testing development with children and interacting with them goes better when they're distracted and happy than crying and non-cooperative (--.-- remember though there will always be children scared of strangers. It's natural at certain ages and at that point they won't be happy to let you do their temperature etc.)
Another is to know what children are into, from favourite games and cartoons of young children to the movies, console games and hobbies of teenagers, conversation is easier on familiar topics and great for building a patient nurse relationship.
Family centered care has been mentioned before already, I guess just remember sometimes it's more the parents you're reassuring/educating/assessing than the child. Welfare...Ward of the state...Failure to thrive cases anyone?
Also to remember kids crash fast! But also get better fast, so turn over is often quick. That Respiratory failure comes first over cardiac failure.
Qualities...I'd say ^.^ Playfulness (when needed), being able to listen (Kids ramble a lot! XD) , patience (Tantrums and naughtiness reign on), and knowledge on development milestones and age needs are important.
Overall though? I think it's that you 'enjoy' it.
If you can enjoy looking after children when at their worst, crying, in pain, missing home/parents, naughty. Yet still find yourself happy at the end of the day, then I think it's the perfect job for you.
I love my job, I really really love spending time with children, seeing them smile, seeing them get better. If you find yourself feeling that way then go for it! The more paed nurses in this world the better!