If your dog is a good boy it might be difficult for them to not be 100% truthful all of the time. This guide will help you explain the difference between "the truth" 😃 and "the truth" 😠.
(if your dog is a cop they have to tell me)
Tell your dog to be cool.
Remind your dog that they are a dog, not a rat.
Ask them if they always do everything their parents tell them.
Try roleplaying with your dog; say that if they were in your shoes that you wouldn't tell on them.
Let your dog know that they're already implicated and you'd both get in trouble if they told on you.
Now hopefully your dog understands the importance of not being a snitch.