For a child that young, you might want to try Dr. Christopher's Kid-E-Reg.

Here is what is said about it:
"This synergistic herbal blend was designed to strengthen and gently cleanse the lower bowel. It is powerful enough for effectiveness and gentle enough for safety. It tastes great and is made with only the finest ingredients. They are as follows: Slippery Elm Bark, Licorice Root, Fennel Seed, Anise Seed, and Fig Syrup. We never use alcohol in any of our children’s formulas—we only use 100% pure vegetable glycerine as a base for all Kid-E-Line products."

I buy at http://www.vitaglo.com/co0120.html which carries all of Dr. Christopher's products at a good price and free shipping over $50. (I am not affiliated with Vitaglo but have bought from them for years)