10 minutes for the cooling. 10 minutes for the electrical system.
You could overview the main components of a car in under a half hour. Adding different versions of the car will add some more time to that, and if you want to do the practical pieces it would fill in the gaps.
Remember, this is a "fire hose approach" you are just pointing out the components, you aren't replacing them. Where I did say "you could have the kids do this", it is very much dependent on the vehicle. Some cars you can change the accessory belts in 5 minutes (let a kid try this). Others take a few hours. Don't do that as an activity). And the activities can be omitted if time is tight.
I've actually done two of these classes before, one to scouts and another to a singles ward RS group. And you are right, not everything is going to be covered in each class, especially if they ask questions. That's just an outline.