My son and I adapted this recipe from Family Fun to our tastes.
1 cup canned pumpkin
1 1/4 cup vanilla low-fat yogurt (we like Stonyfield)
1/4 cup vanilla soy milk (we like Silk Very Vanilla for Kids)
3 tsp brown sugar
1/2 tsp cinnamon
1/4 tsp nutmeg
8 ice cubes
Put all ingredients in the blender and puree until smooth.
Thanks @jules23 for suggesting soy milk - it made an excellent addition!
Love this old pic of T from his second Halloween: