In infancy and toddlerhood, these are perfectly healthy self-soothing habits. They help the child feel happy and safe when encountering a new or stressful experience (which happens frequently, as everything they encounter is new to them). The benefits of pacifiers or thumbsucking are many, both for the babies themselves and for their parents.
However, after a certain age, continuing these habits can change the way the developing adult teeth will come in. It can even change the shape of their dental arches. Most children will grow out of the habit on their own by age 4. If they aren’t showing any signs of stopping by then, it could be time to intervene.
With pacifiers, it can be as simple as taking it away or trimming it down until the child loses interest. Thumbsuckers can be trickier. Nasty-tasting topical aids are an option but they aren’t perfect. We recommend praising successes rather than scolding failures, giving them activities to keep their hands too busy for sucking, and putting socks over their hands to discourage thumbsucking at night.