Do small belly button hernias need to be removed?

No. Small umbilical hernias usually don't cause much trouble, as they enlarge there is more likelihood bowel or omentum will become trapped in the hernia. Usually they can be repaired easily as an outpatient, often with a small piece of mesh placed behind the hole just outside the lining of the abdominal cavityie, the peritoneum.
Sometimes. If it is not causing pain, no. If age under 5, definitely no.