I’ve searched but I can’t see where to turn on “coming soon” for when I do site maintenance?
Help… I’m live, and shouldn’t be. 
I’m pretty sure that you’ll have to install a plugin for a Maintenance Mode. I used to use a plugin years ago, but I currently don’t have use for one anymore. Visit WordPress.org and search for a Maintenance plugin of your choice like https://wordpress.org/plugins/wp-maintenance-mode/ and install and activate it from your WordPress admin. Good luck!
There aren’t any option built in Sparkling theme to setup coming soon page and there aren’t such an options available in WordPress. However, there are plenty of plugins available that will help you with that. Here is one free plugin that you might use.
If you don’t like this plugin just search for others on WordPress.org as there are plnety of great plugins available for this purpose.
Let me know if this helps.