If there is a predictable relationship between the client IDs of the textboxes and checkboxes, then you can use the following:
1. in the ValueChanged client handler obtain the client ID of the textbox with sender.get_id()
2. in the new variable replace "textbox" with "checkbox" - and you now have the checkbox ID
In this way, you will be able to use the same function for all controls and what's more, to move the script in an external file.
the Telerik team