Suggest Treatment For Constipation In A Child

My grandson is 18 months old. he s a mess. my dghtr has tried getting him on a schedule for eating and sleeping, it works ok for 2 days. then he won t eat or drink or sleep. then he gets himself all screwed up. He does have constipation problem, just like his older sister. my dghtr has taken her to the pediatrician, she says that mom has bad habits. nothing wrong w/child. Where should my dghtr start? what dr should she get an appt with first. psych, or digestive, etc.