The dialogue that would best help develop Casey's character as lighthearted and jovial while still conveying the seriousness of her claim would be:
“Lots of reasons. There were nibbled food boxes, the scratching sounds in the wall—oh!” Casey grinned. “And I saw a mouse in the kitchen this morning.”
This choice highlights her playful nature with the phrase "oh!" and the grin, which suggests that she's not entirely serious but still concerned. It effectively balances her instinct to joke around with the need to communicate the situation, helping readers understand her character better.