Two mothers, each accompanied by her son and daughter, want to cross a river.
1) No child can be with another mother on a bank or on the raft without his or her mother.
2) A child can be left alone or with other children, but not with a different mother.
3) The raft can hold a maximum of two people, The mother and her son wearing red clothes are the only ones who can row.
How can they cross the river?