Who proposed making a covenant to send away the foreign wives and their children?

Answer: Shecaniah son of Jehiel

Shecaniah son of Jehiel confessed the sin, said there was still hope for Israel, and urged Ezra to act.

“Then Shecaniah son of Jehiel, an Elamite, said to Ezra: “We have been unfaithful to our God by marrying foreign women from the people of the land, yet in spite of this, there is hope for Israel.”— Ezra 10:2 (BSB)