I looked this up on a whim and found some info:
My kids are in the 4-6 year category here, and it looks about right:
http://www.weightlossresources.co.uk/children/nutrition_calorie_needs.htm#cals
The trick is making the calories count. I'm lucky that my kids LOVE high calorie foods like avocado, peanut butter and bananas, so I can restrict things like pasta and white bread and not worry that they're not getting enough fuel.
Here's another chart:
http://www.bcm.edu/cnrc/consumer/archives/percentDV.htm
My issue is making sure they get enough, and that I'm not too overzealous in my "crap cutting".
My daughter is a skinny mini, and never stops moving. Today she asked me for a hamburger bun with peanut butter on it. Instantly my Nutrition Warden hat came on, and I recoiled at the thought of letting her eat refined, white bread.
"Mommy please? I know the bun has no vitamins, but the peanut butter does, right? Please?"
I gave in, because I knew she'd be back outside in the yard, burning it all off. Sure enough, as I type this, she's hanging from the monkey bars.