I need to have an umbilical hernia repair. Nobody seems to be able to answer whether I should fix it now or wait until I am finished having children.

Depends. If you need to have the umbilical hernia repair because it's causing pain, because it's large, or because it contains intestines, then you should have it fixed now, then recover for several weeks, and then have kids. If the hernia is small and is not causing pain, then you could probably have kids first, however pregnancy could make the hernia larger and more painful.
Depends on severity. Hernias require repair to prevent complications or worsening. If a lump is getting stuck in the hernia and can't be pushed back into the abdomen (reduced) it may contain intestine which could be dangerous. But if it is just an interesting finding on exam and is a little sore, it should probably wait until after last pregnancy since stretching the repaired area may make the repair fail again.
Depend on situation . If it causing you symptoms then you should fix it because it may get worse if you are pregnant because of increase abdominal pressure can worsen the hernia.