I'm sure you've mentioned it already, but don't get all intense with the 'instructions'.
I've seen guys tell their kids 100 different things before junior ever touched the gun. The kid is immediately overwhelmed and thereafter pretty much wants nothing to do with the gun. But then Dad gets impatient and forces the kid to proceed. The whole scene is a disaster.
I've taught many a youngster about guns. The day of the shoot is not the day to 'start'. You should have gone over a few basics a day or two before shooting. This also builds anticipation and interest.
Here's the safety rules I give to kids who are just learning-
1) Keep your finger off the trigger until you're ready to shoot.
2) Keep the gun pointed in a safe direction.
That's it.
Going on & on about whats behind the target and all guns are always loaded and other details can be added as comfort increases.
When it comes time to shoot and hold the gun; this is the safety and this is how it works. These are the sights and this is how you look at them.
That's it.
They've obviously been watching and know how to hold the gun (basically) and how the trigger works. Some fine tuning with holding the gun, and let them shoot.
They're gonna make mistakes. Usually forgetting about keeping the gun pointed in a safe direction. Do not bust their chops and go all adult on them! Calmly repeat about keeping the gun pointed in a safe direction.
Encourage them by relating stories about how when you were that age and just started. How bad you were, couldn't hit anything. And that one time you made a miracle shot. How Grandpa was funny. These kind of things.
Do not be condescending. Do not treat them like a child, but rather a young adult. They pick up on this and act accordingly.
Do not force anything. If they wanna stop after 15 minutes, stop.
You got one chance to make a first impression and this is it. If you blow it, you have years of correction awaiting you.