Ellen, Michael, and Sean establish Chief Brody as an everyday family man devoted to his kids and still in love with his wife.
Sean, the baby, is the personification of innocence and childhood as he plays in the sand and sings, "Do you know the muffin man?" And the scene in which he mimics his dad (how cute is that?) endears Brody to the audience.
Michael, the older son, is the adventurous youth whose boyhood play—and his life—is directly threatened by the shark, propelling Brody to get over his fear of water and set out on the Orca.
And Ellen's flirting, fussing, and worrying over her husband make the audience love, care for, and worry about him too. These three are Brody's insurance policy against the shark, because Spielberg knew that if Brody was killed the audience would revolt.