I'll be running the Empire and my son (10) and any friends we can gather will be running rebels. I want it to be fun for him and his friends, I don't care about winning as Imperials but still the game needs to be challenging for them.

I don't know where to start!

There is a learn to play guide in the core box. Start by reading it, then explain it to your son and his friends. Play the tutorial mission too (in that guide).

If you just want to have fun, but you still want to offer a challenge, don't play subversive tactics and try to lay off of stuns - once in a while is ok, but bringing gards and imperial industry in every mission might be too much.

Read the missions in advance and prepare your voice acting, as the narratives often include dialogues.

Just get to it, play a bit, don't worry about making mistakes. The tutorial is only a tutorial, it is only meant to introduce you to the core game mechanisms. If you intend to play it fully instead of jumping directly to Aftermath, check the FAQ for a small errata to it.

Skim through the rules reference guide so you know when to refer to it. Read through it after the first couple of misions.

There are a lot of beginner threads in boardgamegeek as well. (Easy to find with Google.)

When you have played the tutorial or Aftermath, https://www.boardgamegeek.com/thread/1319188/what-are-some-crazy-or-common-rule-breaking-mistak

Preferably the rebels would control 4 heroes, but for your first campaign, or first missions it doesn't matter that much.

You might want to have a look at our campaign playthrough:

We tried to give rules explainations and insights during playing.

Also we have some rules explainations and how to set up the campaign in Episode #1 and Episode #2 of our podcast:

Here's the video I used to learn how to play, he does a good job:

Also, shameless plug, here's my own One Turn tutorial video where we show a single round of gameplay constructed to show off much of what you can do in the game. It's one of our older videos, but it checks out.

Good luck, and have fun! It's a fantastic game.

The Board Online Board Offline campaign video was fantastic!

Thanks for that!