Stools get darker on taking iron supplements and some kids may get diarrhea others to get constipation. Was your child anemic? Why did the doctor start the iron supplement? Consult with your doctor as to what to do now- stop the iron supplement or start probiotics?
Stools don't darken on changing milk brand but they may turn runny on it.
For further consultation, you can choose from our list of best Pediatricians in Islamabad and seek in-person advice.