Why not keep it real when photographing kids with props that show their true passions? As a parent, it's fun watching your children grow and develop their own personality and interests. One of my own passions is photographing my own children (go figure). Of course someone who isn't his parent might quickly deduce that my son (pictured here) loves tools or building. Everywhere we go, he insists on bringing more than a dozen plastic tools, his safety glasses, his work helmet and a toolbox with him, so most people usually take notice and say something to him.
When I had the idea to do this shoot, I told him about wanting to use real tools. Did I mention that he only turned three last December? So, his eyes opened real wide and he couldn't wait to get started. And once we did, he had a blast with some silly tool-talk. There's no fakery here, it's all real. There's also no reason to say "smile" or "say cheese" to get a smile. He had every reason to smile. 
So, a general tip is to keep it real. Why use generic props that just anyone could use? Of course for the little princess, a legit diamond encrusted tiara might be a bit hard to come by.