Thanks for asking.
Based on your query, my opinion is as follows.
1. Low fiber in diet will cause hard stools.
2. Due to hard stools irritating the colonic mucosa, increased mucus will be seen.
3. Balanced diet is necessary for all ages, particularly childrens. Discuss with them and make sure, your grandson receives a balanced diet. Yogurt has not complications, however in excess can cause diarrhea
due to mal-absorption.
Hope it helps.
Any further queries, happy to help again.