Is it just because my son is young (12) that it seems to be an item of chance if they could see a hiatal hernia on an endoscopy?

Age should not make any difference in being able to diagnose a hiatal hernia.

I do remember the doctor saying something about how strong a child's sternum is while we were disussing this, but did not understand what that had to do with seeing a hiatal hernia.

Nothing I can think of.

My confusion is why anyone would go through that proceedure to check for a hiatial hernia if he seems to have such a high chance of not being seen? And, everyone spoke as if it would be conclusive. I can only figure this has something to do with adult vs. child.

I don't see why age would make a difference.  But small hernias can be missed, or it is possible that the hernia is intermittent so that it may not have been present at the time.

When you speak of the x-ray and barium, do you mean what they refer to as an upper GI series?

Yes.

And is that truly conclusive?

For the most part, but there can still be issues as I mentioned above.