The functions you are creating should be either FRIEND or PUBLIC.
In the aspx file, you will have an object of the user control accessible via the ID of control on your aspx page. Using this object, u can access your functions.
As far as validation is concerned, it should be on the control itself. This is because, as you said that your control is a composite control[has many child controls], but on the aspx page all those will be treated as a single object. The individual validation for each child control in your user control can only be implemented in the user control itself. And it is the recommended way !!
Reading the control values goes the same way, instead of functions, implement the values a s properties, in ascx, then call those properties from aspx.