Try to show and explain how smoking pot can keep them from being successful. Drug testing is very important with most companies these days. If you fail the drug test it's over. For a young person it may be best to introduce the scared tact. You may want to read something so fast to them and asking them to give you the main topic of what you read. This could help you prove short term memory loss, if you do it when they are not high.
Either way, good luck as a parent it's easy for us to blame ourselves for our kid’s short coming.
Hopefully, this is just a phase at six years old.