Cleaning the Battlefield

Alex suddenly stopped and squinted at the long corridor not far away. He came in through the side door. Along the way, there was a thick layer of dust on the ground. There were no other footprints. However, there was a mess of footprints in the connected corridor in front of him. According to the size, two of the footprints should belong to a little boy. He was about eight or nine years old. The other two were girls, about four or five years old… So Mia and the others had been here?