It's so important to tell children they can do nothing and should do nothing to stop climate change... yet. Their primary responsibility to society is to educate themselves. Until they have a job and an income it will be difficult for them to either understand or change the status quo. It is a disservice to fill them with activism while they are, frankly, not capable of understanding what they are up against.

Alternatively, start your speech by saying you have a boring talk with lots of dry facts but that they can vote from four other topics for you to talk about instead. Like, how to make a million dollars or how to find the love of your life or what video game is the best or what to watch on Netflix. After you have them vote, point out how easy it was to distract them from the topic of climate change, and ask them why they think leaders and adults would be any different.