Sprout Forms: creating additional text inputs in radio and checkbox labels
The best approach for Radio Button and Checkbox Fields with a Select Other option would be to create a custom field (or fields) using Sprout Forms Front-end Field API.

You could use the existing Radio Button and Checkbox Fields as a starting point and adapt their UI's to present your users with the Select Other option.

You can review the front-end field classes in the Sprout Forms plugin here:


sproutforms/integrations/sproutforms/fields/SproutFormsCheckboxesField.php
sproutforms/integrations/sproutforms/fields/SproutFormsRadioButtonsField.php


It's on our list to add support for front-end fields with this behavior, but it will likely happen after Craft 3 is released. Please reach out if we can help with answering any questions regarding how to implement custom Front-end Fields or custom development support.