Sometimes, this happens in children who have a weak anal sphincture which is unable to hold stools in the rectum and when ever there is any straining activity or emotional stress, small amount of stools may come out and soil the underpants. It requires pelvic exercises and evaluation of anal muscle tone as well spinal examination. It is also seen in cases of behavioral constipation. The child is to be made to go to toilet regularly and daily whether he has or not any urge to pass motions. Also, laxatives like Syrup Liquid paraffin to be given every night about 10ml till his toilet habits become normal and there is no constipation. Also avoid foods which cause constipation, like excess of milk products, dry fruits, high protein fatty diet and bakery products. It will be better if you can see a paediatric surgeon for the same.