The easiest way is to use the page's ClientScript property. You can register some code to run when the page loads using something like
ClientScript.RegisterStartupScript(GetType(), "hiya", "alert('hi!')", true);
See http://msdn.microsoft.com/en-us/library/asz8zsxy.aspx for more info.
This should be available from a child control by tacking "Page." onto the beginning of the code above.