Cpl. Steve Case, commander of the bomb squad, said it took his crew about 10 minutes per egg to wire a switch to a nine-volt battery and a small speaker.

Case said they’d been building the eggs for about five years but had always given them to special education schools and the like, who often hosted their own hunts.

“We never get to see kids find eggs,” he said. “This makes it all worth it. All the burns from the soldering irons.”

Over the afternoon, some children smiled who hardly ever do, parents reported. An autistic child launched her hunt early, because she just couldn’t wait for the official start, said Juley White, a St. Charles County park ranger. And parents didn’t have to apologize for meltdowns.

“That’s so empowering,” she said. “To have the support of other people who understand you.”

The hunt was still hard for Evan, despite the chirping eggs.

Other children ran from egg to egg. A girl found them with her cane, announcing the discovery each time. Some meandered more cautiously.