Police said the 10-year-old boy died immediately after the explosion on Tuesday and his 9-year-old sister died of her injuries in the hospital on Thursday. The children were from the village of Konskowola in eastern Poland.
Experts are trying to determine the age of the explosive device.
Poland saw many battles between the Soviet and Nazi German armies during World War II. Bombs left unexploded after the war ended in 1945 are still discovered from time to time.