Heh... I'm sure you've heard the poem about the person who was sincere but mistaken on that issue:
Here I sit, broken hearted
Paid a dime but only farted.
Next time I won't take that chance
Save the dime and **** my pants
In the same vein as your scenario, would you treat it as CERTAIN that your child's claim is true?
For instance, say in your scenario, you dutifully take your kid to the washroom, but 5 minutes later, he says he has to go. Do you say to yourself "maybe he didn't actually go the first time" and take him again, or do you say "no - you said that you had to go 5 minutes ago, so I KNOW you don't have to go now"?