I had that problem when my daughter was 3; I was really worried and bought prum juice, probioticos and anything that would help; we used bubbles in the bathroom to distract her; I decided not to worry as I would pass the stress to her; but if I noticed she needed to go I would take her and seat hear in the toilet and used to tell her poop needs to go out or your tummy will hurt and you can get sick. She eventually got it.