We have a handout and protocol about constipation on our website here. I realize there are more products available on the market. One website that I think is helpful for parents and families is the Pedialax website. It has good information on their products and how to help kids with constipation.
I have had confidence in the safety for long-term use of propylene glycol ("PEG"; Miralax(r) by Merck) over the years. In fact, one of my sons uses it regularly (almost daily) to keep the bowel movements regular. The head of the gastroenterology department at Nationwide Children's Hospital, Dr. Carlo Di Lorenzo, M.D., is quoted in an article from the New York Times. I feel comfortable with children being on the medication long-term at this point. Having said that, further research needs to be done to ensure the safety. Ask your pediatrician if you have further questions.