Client Side Script With Server Side Controls

I have a web form that has a checkbox on it and a textbox associated
with it. I want the textbox to be displayed when the checkbox is checked
and I want this to happen on the client-side instead of posting back to
the server. The checkbox is a server control (asp:checkbox...). I can
write some codebehind that will display an alert message upon checking
the box that looks something like this...

Page_Load...

Dim s As String
Dim scriptString As New System.Text.StringBuilder

chkExpDate.Attributes.Add("onclick", "alertmsg()")
scriptString.Append("<script language=JavaScript> function alertmsg()
{")
scriptString.Append("alert('hi'); }<")
scriptstring.Append("/" + "script>"
s = scriptstring.ToString()

Page.RegisterStartupScript("startup", s)

End Sub

I used this as a test to make sure the server control was running the
script. But, when I change the script to assess the value of the server
control checkbox such as checked = true, I get errors and if I just try
to display the asp:textbox, it doesn't work either.