I agree with Portia, that the key is the subject matter. You're not going to find good things studying the Holocaust, slavery, feudalism, the Crusades, etc.
My DD has recently been exploring the history of the Apollo program. Apart from the tragedy of Apollo I, it's a story of triumph. Very uplifting stuff. History of the sciences or philosophy are generally pretty good topics, as long as you don't spend too much time on what happened to Galileo, for instance.
Certain violent topics can be framed in an positive way, because of what was accomplished. The American Revolution. The US Civil War. World War II. In order, they ended tyranny, slavery, and more tyranny.
And even in some topics like the Gold Rush (I assume you mean the California one here, as there were quite a number of them), the devil is in the details. When you look at the big picture, you can show a lot of positives... many people getting rich, new towns being born and permanently settled, etc. It's only when you get down in the details of daily life that it starts to get ugly. So, maybe the solution there is to stick to materials that are more general. That's actually quite common for history resources targeted to elementary age children.
