Do they know?

The cellar room they were hiding in was cold and dank and it smelled faintly of pickles.
“Are we going to stay here long?” asked Ruth, the youngest, her eyes wide and scared.
“I don’t know,” her brother Isaac said. He sat slouching against the wall, knees hugged to his chest. “We’ll just have to wait and see.”
“But what if they find us?” Ruth’s voice quivered.
“They won’t,” said Isaac.
“What if they already know about us?” whispered Madelene.
“They don’t.” Isaac sighed.
“How can you be so sure?” snapped Madelene.
“If they did, we’d all be long dead.”