Assuming that you have Username value in Master Page.
This is the sample code i give you.
Write this code in the Master Page Load Event. And set the Cookie value.
protected void Page_Load(object sender, EventArgs e)
{
HttpCookie MyCookie = new HttpCookie("UserName");
MyCookie.Value = lblUsername.Text;
MyCookie.Expires = DateTime.Now.AddDays(1);
Response.Cookies.Add(MyCookie);
}
And to retrieve data from cookie in child page
In the Page_Load event write
protected void Page_Load(object sender, EventArgs e)
{
string userName = Request.Cookies["UserName"].Value;
}