Up to a cap (17 gr or 1Tbs) in 8oz of fluids, 2 times a day. That is about the max though, and will make very soft stools (which would be needed if you are trying to heal mega colon or mega rectum issues).
Miralax is safe to use as it doesn't stimulate bowels, and therefore won't cause lazy bowels.
Alexa, I agree with Amy, don't be quick to judge, some kids have major medical issues warranting the use of Miralax for a really long time. My daughter was on it in for 4+ years.