Answer me this from Number the Stars, Chapter 11 by Lois lowery. What does Peter give to the baby and why?

Peter gives the baby drops to make her sleep.

Peter reached into his pocket and took something out. He went to the parents and leaned down toward the baby. He opened the lid of the smal bottle in his hand.
"How much docs she weigh?" Peter asked.
"She was seven pounds when she was bom," the young woman replied. "She's gained a litle, but not very much. Maybe she weighs eight pounds now, no more."
"A few drops wil be enough, then. It has no taste. She won't even notice."
The mother tightened her arms around the baby and looked up at Peter, pleading. "Please, no," she said. "She always sleeps all night. Please, she doesn't need it, I promise. She won't cry."
Peter's voice was firm. "We can't take a chance," he said. He inserted the dropper of the bottle into the baby's tiny mouth, and squeezed a few drops of liquid onto her tongue. The baby yawned, and swalowed. The mother closed her eyes; her husband gripped her shoulder.