If you added the checkboxes to the secondary data source then I imagine that's why they're greyed out. I think they might be read only or locked controls.
I guess I don't understand the scenario. For cascading dropdowns, the user makes a selection in the first dropdown. This triggers a rule or a filter that then populates the second dropdown. The user then makes a selection in the second dropdown, and they're done. The dropdowns are bound to fields in the main data source (but they may be receiving data from a secondary data source, of course). I don't really understand how the checkboxes fit in to the cascading dropdown scenario. Can you explain? Feel free to add attachments (screenshots, forms) via the Options tab when replying.
Thanks!