Can I use that time I was published in the local paper in 3rd grade as a publishing credit now? I entertained the idea, because it put a smile on my face and almost made me laugh aloud. However, the more I explored the "yes" and "no" to this question...the less sure I became. I mean, I was in the third grade 11 years ago. Some authors, I'd imagine, reference their credits that are older than that. Surely that wasn't their best work and they've grown since then. Perhaps not comparatively to the degree of a child, but still.
Pub credits are works that have been published by someone who chose your submission from a competitive field and generally speaking offered you a contract for publication.
Publishing anything by a third grader in the local newspaper is considered cute, not competitive.
If you put that in a query letter and I discovered it was something you wrote when you ten years old, I'd stop taking you seriously.
Don't reach for pub credits you don't have. If you don't have them, you don't. I've signed and sold a LOT of writers who had zilch on their resume other than "I want to be a published writer" and were smart enough not to say "I've been writing since I could hold a pen" or "I've wanted to be a writer from when I was in kindegarten."
A query letter is a business communication. Don't do anything that makes you look silly unless you are writing the next Captain Underpants.