When you are hiring a developer to develop a (child) theme you may be asked for admin access to your WordPress website. A few options will come to mind, you can create a new account and give him login details or hand him access to your account, but what is really the best thing to do?
You could have him setup a new site for you
You might want to create a separate copy of your site. This way you can have your developer work on a non-live version of the site and it will not interrupt the user experience. In the WP community we actually call developers who work on live sites “coding cowboys”. It’s really not the best way to go!
You should have the developer work in their own digital environment
The best thing to do would be to have them develop on their own domain and when it’s ready then they can just send you a backup of the site or upload it live without much interference for the user!
If you do decide to give complete access then be VERY careful. If the theme + plugin developer is not qualified then you risk having all your work and content ruined. Any seasoned developer will always make a full backup of the site before changing any code!
A developer can also do most of the work offline, but I think its better to do it on a live server configuration because by using the same configuration you can be sure that everything will work very well!