So, what are your goals, to give a great talk?
Or to change the financial history of the next generation in your family?
If you want to have the next generation change/or improve/establish good financial habits - i.e. "behavior change" then one lecture isn't going to cut it, in fact, a lecture format will likely have no impact.
Imagine everyone in your country, community and family is fat. You give a lecture explaining how you cut calories and started to exercise and lost and maintained weight. However, those people like donuts and binge watching Netflix. After the lecture, many people may start eating veggies for a week, or they may buy them at the grocery store and watch them rot in the fridge. They will likely buy a bike with intentions of riding it, buy a tread mill for the same thing, or get a gym membership - but after a few weeks, with their busy schedules, they will go unused.
So you are up against two things: individual behavior change as well as community pressures. If you do not have a long term plan to address both of those, your lecture will have no impact, even if you are super charismatic and good at presentations, which is sounds like from the "spreadsheet" suggestions and such that the lecture will not be.
I would recommend that you make concept - financial independence/strength for the next generation of your family - a long term project in which they only participate in if they jump through hoops. For example - it MUST be practical - like a really fun hook, then they must have some sort of committment/investment on their part. So keep the intro talk very general and fun - the basic article that was in the news about retiring before 30 - then working only with those who want to do it on individual plans where they support one another. Some will fall off the bandwagon but that is OK. Some won't start at first, however, will see the success of others and want to join. You will need regular social support (this is why there are support groups for difficult things like lossing weight (Weight Watchers - people think it was successful for counting calories, but everyone knows you need to count calories, it is the weekly accountability and social support that defines it) and Alcoholics Anonymous.
Those young adults are like donut and Netflix addicts. To get them to give up their iPhones, foreign vacations, leased cars, eating out is going to be difficult. I would do things like start a competition:
1. Everyone joins some sort of investing portfolio of their individual choice, and have the competition be not who make the most profit, as that fluctuates, but who socks aways the most $$$ for the year while they learn about and explore the world of investing
2. Variations of educational and practical - do not teach any subject without doing the practical - i.e. don't teach about investing, help them to get started investing. In the U.S. we thing about teaching a lecture. You may want to get them with a 30 second hook on compound interest, but go the other way - get a group of them together who want to invest and have the group help each other.
The steps to behavior change are debated, but about seven steps. The first is awareness (most Americans are not aware of how stupid they are with money), the second is knowledge. Most people do not get past step #2, knowledge, despite good intentions to stop smoking, go on a diet, or get financially stable. You have five more steps to go through, and they are much harder that step #1 and #2.
I would also not leave the leadership to this to yourself. I would give them ideas and incentives for them to organize themselves and have you act as a motivator, advisor, catalyst, and partner. Only work with those who are interested. Don't worry about the others.
I wish so much that my family had done this with me. Beginning with helping me learn how to do my taxes when I started working. I even asked but I don't think they had the knowledge to do so. Having a mentor years ago would have helped, but I'm not complaining, just commending you for helping the next generation of your family.
If you set it up right they will come to you as the "maven". They will all be starting jobs and wondering if they should join the 401(k), buying new cars, getting mortgages, etc. over the next few years. If you set yourself up as the cool older relative who can help them with those things, then they will all be coming to you for advise.
What you are doing is amazing and I really commend you for it.