Thanks for your question regarding SSL certificate for addon domain
You may purchase an SSL for an addon domain just as you would any other SSL certificate, however, keep in mind that you may only have one SSL certificate valid per cPanel account. For more information on purchasing SSL certificates, you may view the following article:
If the SSL is applied to the main domain does it apply to subsequent add-on domain (since they are sub-directories of the main domain)? Like one certificate for the parent applies to the children also?
@Forrest. Not exactly. There are "Wildcard SSLs" that are meant to work for new subdomains. However, the SSLs we resell through Comodo can only work for one top-level domain or subdomain. However, subdirectories under one domain will be encrypted.