Ideally, teaching discernment to our children begins at birth! Everything we do and say instructs and models something--whether we're aware of it or not.
But there are more formal methods out there for teaching discernment--as soon as your young child can talk. What are these methods? Very simply: instruction in Biblical literacy! What better way to teach discernment to our children? After all, bank tellers learn the real money in order to better spot the fake money. The same principle holds true here--our children will learn truth in order to better evaluate and spot potential falsehoods.
I'd love to start a mini-series on this idea and highlight some resources that have aided us. Our children are still quite young (4-year-old twins and an almost-6-year-old; Megan's oldest is 4) so I/we don't have much experience with the resources out there for older children. Feel free to weigh in with your thoughts in the comments! I'm sure Megan will contribute, too, but she and I have used many of the same resources--so rest assured that much of this content is coming from both of us even if I am the "poster."