You shouldn't need to create a subdomain in AWS. As soon as you configure your DNS records correctly for all subdomains to point to the right IP address of your server instance. Now, on your instance you may use a reverse proxy like nginx to dispatch the requests to a correct Node application because this will be easier than implementing that logic in your Node applications and will let you avoid tightly coupling of the Node applications and their current subdomains.

For details on the proxy configuration follow the links in this answer:

• Django and Node processes for the same domain