The most likely reasons for your toddler's constipation are:
Eating too many low-fiber foods. If your child eats lots of milk, cheese, yogurt, or peanut butter, for instance, and not enough fruits, vegetables, and whole grains, she could wind up constipated.
Toilet anxiety. If your toddler is feeling pressured about toilet training, she might start deliberately withholding her stools. If she shows all the signs of straining to have a bowel movement — stiffening her body, arching her back, and getting red in the face — but nothing comes out, she may actually be trying to hold it in.
Even if your child is potty-trained, she may not be taking enough time on the toilet to completely empty her bowels. That can lead to a buildup of feces that causes the colon to stretch and cramp. An enlarged colon can lead to larger-than-normal, difficult-to-pass stools, making your child even more reluctant to use the potty.
Dehydration. If your toddler isn't getting enough liquids, her system will respond by absorbing more fluid from whatever she eats or drinks — and from the waste in her bowels, as well. This can result in hard, dry bowel movements that are difficult to pass.
Lack of activity. Movement helps blood flow to your toddler's digestive system, so if she's not active, she may experience trouble in the BM department.