ELK RIVER, Minn. – A man rescued his two children in trouble swimming off a Mississippi River sandbar and then disappeared in the swollen river himself.   This about 11:45 a.m. 20 miles upriver from the north Minneapolis suburbs A river rescue team began a search for Cody Jay Pope, 42, of the north St. Paul exurb of Circle Pines. Sherburne County deputies said Pope had been in the river  when two children began struggling in the strong current. He grabbed one child and brought her to safety on a sandbar. He went back to assist the other child and was able to support and push the child to shore before he himself disappeared in the water.