Fly fishing is a great activity you can do with kids that will leave a lasting impact.
Outside Online said to start small, "Tie a big, bushy fly on the end of the tippet and clip off the hook, then cast with your hand over your child’s until they learn how far forward and back the rod should go—the classic ten-o’clock, two-o’clock motion. Set a dinner plate on the grass, and see if your kid can hit it."
Then move up to a calm body of water and try again! It's alright if they don't get it right away, really fly fishing is about the calm, the company, and the memories.