I know I'm very late to this conversation, but my son (17-months-old) actually takes three doses of Miralax a day to keep him cleaned out. We do 12 grams when he wakes up, 12 grams 4 hours later, and 3-4 grams 4 hours after that. (A small dosage change seems to go a long way for him, which they say isn't supposed to be the case. We played with the dose a while to figure out what works for us.) The other thing with Miralax is to make sure he's getting a lot of extra fluids (water, juice, milk), since it only really works if you're well-hydrated.
