One of the big signs of micronutrient deficiency, and the first sign this is working is the “bowel change”.
It is probably because the first muscle to get stronger is the intestine.
For the love of all that is holy, please don’t use Miralax on your child. According to the FDA it is intended for not more than seven days use because after that it interferes with the lower intestines ability to absorb micronutrients.
If you have a child with a micronutrient deficiency issue, the last thing you want is to make it impossible for them to absorb what they need from their normal food supply.
Getting your child off of Miralax (or other osmotic laxative) is going to be challenging. DO IT ANYWAY.