I've taught first aid to quite a few classes of Cub Scouts over the years. I pretty much go through the Readyman activity section in the Webelos book. It is written at an age-appropriate level and is easy for the kids to understand.
It covers the major emergencies an 8-10 year old can be expected to understand:
Heimlich Maneuver (choking)
Always stand near a wall when in the presence of Cub Scouts who have learned the Heimlich Maneuver. ALWAYS. Especially if you have food or drink in your hand. If you cough, even a little, one of the Cubs will promptly "rescue" you. EVERY time. It takes a while for them to learn to ask whether or not you are actually choking.
They are fascinated with blood. Some to quite a disturbing extent, but you can't fail to get their attention if you use fake blood when you demonstrate how to stop bleeding. I decided to use Bob Amick's "B" recipe for my demonstration. As he says, it flows well. You can tint it darker for venous blood and brighter red for arterial.
The Cub Scout handbook doesn't discuss arterial bleeders, but I decided to include it in the demo because it really gets the kids' attention -- and because I managed to puncture an artery in my ankle when I was about 7 years old. It was a very small, but well-placed laceration and the blood flow was quite impressive.
I always start with the demonstration of venous bleeding, because it's included in their books. Also because it pales in comparison to arterial bleeding. Most Cubs have already learned the concept of direct pressure by the time they get to my demonstration. For venous bleeding.
Their jaws just drop when I demonstrate arterial bleeding by pulsing the "blood" using a relatively small syringe and small-bore tubing. I tell them about my injury and assure them that a relatively small arterial bleeder really can shoot blood 4-6 feet. I'm not sure they believe me when I assure them that it is possible to at least slow down the flow of a small arterial bleed using direct pressure.
I'd like to be able to tell them that I stayed calm and handled things myself, but the truth is I started screaming at the top of my lungs panicked. Fortunately, our teen-aged paperboy had some first aid training. He was able to control the bleeding before my mom saw it. She's never been very good with blood.
Basic rescue breathing is all the Cubs learn. Some Webelos learn CPR and virtually all Boy Scout troops require it. Mostly I stress calling 911 - to get help and because the operators in our area are trained to walk people through CPR if it's needed.
Again, call 911 - and round up any containers.
I review this one in much more detail. I have the kids list the symptoms. They're surprisingly good at that. Then I tell them that there is one more symptom they've missed. After a little guessing, I tell them that denial is very common and that they need to be aware that an adult having a heart attack will probably insist that they are "fine" or "just having indigestion" - or some similar explanation for their discomfort.
I tell them that sometimes it is important to disobey an adult and this is one of those times. If the adult looks sick enough to frighten them - and especially if the adult is having symptoms on the list for heart attack - they should call 911. No matter how insistent the ill adult is, they should call anyway.
Inevitably one of the Cubs asks if they won't get in trouble for calling when they've been told not to. I assure them that the paramedics will back them up. Even if the adult isn't having a heart attack - even if they aren't seriously ill -- any adult who looks that sick needs to be evaluated by someone other than a child.
Every single time I have done this, someone in the group shares a story. One dad survived because a member of his family had disregarded his assurance that he was "fine". One of the Cubs shared that a grandparent had survived a heart attack because one of the adults present had disregarded his refusal of care. The saddest story was from a young man whose grandfather had not survived. The boy wasn't alone with him, but the adults in that instance had hesitated to call. It might not have made a difference, but he will always wonder.