Matthew 16:19 (13-19) and 17:1-9. I really like the talk the manual refers you to but without pictures my kid probably won't sit through the first few paragraphs of it. But if you want to talk about priesthood keys, have your child help you unlock things and then say our Prophet has all the keys of heaven.
For me though i think I'm going to focus on "faith can move mountains" Matthew 17:20. we're going to make dirt mountains outside and take a spade and say, "this spade is like having faith in Jesus Christ. with faith we can move mountains" and then use the spade to move the "mountain". we might adapt and use mud but even if the dirt falls off the spade the fact still remains you can move the mountain.
gospel library app, children, new testament stories chapter 32, 33, and 34