You can use findControl to find the textbox within a user control, like this (assuming you have a textbox called tb1 in a userconrol called uc1:
me.uc1.findControl("tb1").text
The better way, though, is to expose a public function that can be called to return the value:
In UserControl, create a public function:
public function getValue() as string
return me.tb1.text
end Function
Then you can access it from any page or control that has a reference to the user control:
dim whatsMyName as string = me.uc1.getValue()