How to get span tag inside a div in jQuery and assign a text?

Try this:

$("#message span").text("hello world!");

See it in your code!

function Errormessage(txt) {
    var m = $("#message");

    // set text before displaying message
    m.children("span").text(txt);

    // bind close listener
    m.children("a.close-notify").click(function(){
      m.fadeOut("slow");
    });

    // display message
    m.fadeIn("slow");
}