The first piece of advice I will offer is to avoid giving any stool softner products (like Miralax). Use of these products help people poop by loosening the bowels. Soon you will either become dependent on it to poo, or your bowels will loosen and never tighten back. Then you will have a new problem - never being able to hold it, even as an adult!! Be careful.
The BEST thing to do is give any foods with natural fiber and plenty of water. Fiber is nature's broom for the colon!
Some high fiber foods include black beans, chickpeas (garbonzo beans), peas, bran, oatmeal, 100% whole grain breads (make sure it says 100% WG and not just 'made with' WG), plums or prunes, artichokes, Brussels sprouts, Asian pears, quinoa (great substitute for rice), and nuts.
Here's a link from Mayo Clinic with fiber grams listed for foods: http://www.mayoclinic.com/health/high-fiber-foods/NU00582
Here's one from kidshealth.org about Fiber and Your Child: http://kidshealth.org/parent/nutrition_fit/nutrition/fibe...
Hope that helps! :)