We love Miralax! And it just got deregulated, so it's only $15 at Walgreen's!
Our 4 1/2 y/o used to have 4-6 bm's a day until he was diagnosed last September; the enzymes send him more towards constipation (gotta figure out his dosage AND deal with age-appropriate eating issues), so the gastroenterologist (sp?!) recommended Miralax. Love it. Although Jod's right, of course, about trying to incorporate the more fibrous foods naturally.
Good luck!