4:7 ye shall answer them. It is always true that parents should answer the questions of their children, especially those concerning spiritual matters.
4:9 in the midst of Jordan. There were two markers established, of twelve stones each, one in the midst of the river where the priests had stood, one on the west bank in Gilgal (Joshua 4:9,20).
4:13 About forty thousand. The total number of “men prepared for war” in these two and a half tribes had been counted as about 110,000 (Numbers 26:7,18,34). The 70,000 or so left behind were apparently staying to guard their women and children.
4:22 on dry land. Note that the children of Israel did not have to cross the Jordan on its bottom mud or in shallow water, but on dry land. This was indeed a supernatural miracle. The same had been true at the Red Sea crossing (Joshua 4:23, Exodus 14:21,22).