“Promise not to tell.” Justin hushed his loud voice as best as he could. “I have a top secret.” He looked over his shoulder, as HeHaw, his toy guitar, banged against his hip. Sharon stopped walking, bent down face-to-face with Justin, and rolled her eyes.
“It would only be a first-grade secret.” [Who said that? If Sharon, move the previous sentence into this paragraph.] [What's a first-grade secret?]
When Justin loses words, his guitar helps to find them. [A very odd statement. Whose POV are we in?]
Justin strummed HeHaw and boomed his loud voice into song.
“You’re only gonna get one chance.
You’re not such a smarty pants!”
Struuummmm. He finished his song in the deepest voice he could muster, “You’ll be sorry.”
Sharon looked at Justin, opened her mouth to say something, then changed her mind. When they reached the shade of the mimosa tree, Justin plopped HeHaw on the ground. Sharon leaned against the tree and hung her art case on a twig from a low-lying branch. [A twig
supporting an art case?]
“Your secret, is it something bad?” Sharon picked at the bark and pretended not to care.
“Nope. But it’s big.” Justin kneeled to the ground and reached one hand up and the other hand down. “Bigger than all the Chuck-E-Cheeses put together.”
It's tempting to suggest deleting the four lines that involve the song, because they're interrupting the more interesting conversation. But perhaps in context they'd be less obtrusive. Perhaps this singing commonly happens with Justin.