The following are the common approaches to create a site
definition with sub sites.
Note : You can also
create your own Provisioning Provider by creating a class that inherits from
Microsoft.SharePoint.SPWebProvisioningProvider and then overriding the
Provision(SPWebProvisioningProperties) method. Internally you would then use
the SharePoint object model to add the webs via Web.Add().
Custom Provisioning Provider
Build a custom site definition which has child sites
SharePoint List Template IDs and Site Template IDs
Sharepoint 2010 site definition one template to create
multiple publishing pages
SharePoint Site Definition for Multiple Subsites Creation
Custom Site Definition Sub Site