The "secret omen" the children had been waiting for is a shooting star, signalling their ability to return to Egypt. In my edition, this occurs on pg. 96.
Here's an excerpt:
"A shooting star!" Everybody repeated it in whispered unison as if they'd been rehearsed. Then everybody looked at April. She nodded. "The secret omen," she said slowly, making every syllable heavy with significance. Marshall started turning around and around, smelling the air.