Add this JavaScript function in the head section of your page:
function openChildPopup(id)
{
window.open("childPage.aspx?id=" + id,"myWindow");
return false;
}
Add this code inside your GridView's RowDataBound event:
if(e.Row.RowType = DataControlRowType.DataRow)
{
string id = YourGridView.DataKeys[e.Row.RowIndex].Value.ToString();
Button btn = new Button();
btn = (Button)e.Row.FindControl("yourButtonID");
btn.Attributes.Add("onclick","return openChildPopup('" + id + "');");
}
Now on your childPAge.aspx use querystring to access the details.