Tonight with your 9 to 12 year old
The prompt
“Was there ever a time you told the truth about what happened and nobody believed you?”
What to listen for
If they can describe this clearly, it's likely still a sore spot, being disbelieved when honest is one of the more painful things at this age, so take it seriously rather than relitigating who was right.
If they seem to still be upset about it, just acknowledge how frustrating that must have felt, validation matters more here than solving it retroactively.
If nothing comes to mind, that's fine, but stay alert, since this is a good question to revisit if a real situation comes up later.
What this tells you
This shows whether they currently trust that their honesty will be believed, which shapes how open they'll be with you.