A knocked-out tooth. If your toddler falls hard on his face, call your dentist pronto – your kid may need a spacer to keep the rest of his teeth in place until the permanent tooth appears. White patches. Cottage cheese-like spots on the tissue inside your toddler's mouth are probably thrush, an infection caused by yeast. They usually go away on their own, but if the pain prevents your child from eating, see your pediatrician. Blisters and bumps. These are likely cold sores, which often make their first appearance in toddlers who have shared a slobbery toy with a kid infected with the herpes simplex virus type 1. Check in with your doctor if they're big, painful and oozing or accompanied by fever. Teeth grinding. Kids tend to do this as their mouth grows, and most outgrow it before damage occurs. Grinding can be a sign of allergies or fluid in the middle ear, so schedule'a check-up if you suspect either is the culprit.