3 years old, then it's real safe enough to drink "sugar cane soup" - carrot, water chestnut, sugar cane, yuen sai, meow gun (sorry, I don't know the english names for the last two items. anyway, if you ask the vegetable store in market, they know how to boil this soup. **add the "yuen sai" 15 minutes before the soup is ready.
also, I give my kids one cup of orange juice everyday. Or you can try to change other milk brand. My experiences is that 美贊臣 is better than 惠氏. 美贊臣's milk powder is finer than 惠氏, it's then easier to be absorbed by kids and cause less chance of 便秘