See the section Parent 1 information - Click on the button and a new section appears with more fields to add in second parents info. I know how to use conditional logic, to show the new section but how do I link that to the button as the html field doesn’t show up as a section to apply the CL to??
That’s not something you can do using GF built-in features, I would recommend you to take a look to the following third-party add-on that I think could fit your needs: https://gravitywiz.com/documentation/gravity-forms-nested-forms/
Another way could be to use a checkbox field, and style the checkbox like a button or image where they click it (checking the checkbox) and the next section is then show. The checkbox field will be available to your for conditional logic on the section break.