When is the best time to tell kids now about the facts of life? Is there an app for that?

Start early. Kids want to know the facts of life early. Rather than engage in fantasy (the great stork brought you!) it's better to start honestly but simply. As they get older you can fill in more gaps, and get more detailed. Most kids understand a rudimentary explanation at 4-5 years old and by 1st or 2nd grade may already be telling jokes with sexual undertones. By 10, you're too late.