“ Your wives, your little ones, and your cattle, shall remain in the land which Moses has given you in Beyond Jordan, but you shall pass over before your brothers, armed, all the mighty men of valour, and shall help them.”
This was what they had themselves proposed. The ‘all' was not necessarily to be taken literally. It would be expected that some guards would be left both to arrange for protection and to assist in necessary tasks. And the older men would not be required to go. They were no longer reckoned as ‘mighty men of valour'. This would be a task for the younger men in the prime of life.
But the majority of their men of fighting age (forty military units - Joshua 4:13) must accompany the invading party, for they were part of the tribal confederacy. Israel were a confederacy of twelve tribes bound together by the covenant with YHWH and worship at the Tabernacle, the central sanctuary. ‘Before' means ‘in the presence of, together with'.
“Beyond Jordan”. This was the official name given to land east and west of Jordan used at the time of writing, and probably the name by which it was already known by the people of the land. Compare ‘Ebir-nari' (Beyond the River) a province of the Persian empire (Ezra 5:3; Ezra 5:6). Using it need not mean that the speaker was on the other side of the river. (Just as today we might speak of being ‘in Transjordan').