SharePoint Site Template has one very handy feature – it can be registered inside a Site or you can restrict certain sites to use only certain templates.
It’s obvious how to do this for site collection, because UI provides you specific links, but it’s a bit tricky to assign templates for a Site, so do the following:
1) Upload an existing site template to the site template gallery (Site Settings –> Modify All Site Settings –> “Site Templates") link under the "Galleries" section. Alternatively, you may also use the “stsadm -o addtemplate” command.
The next step is registering uploaded template available to individual sites. (for example Site A can use template X, but Site http://A/B can use only template Y)
2) Open the following link in browser http://<yoursiteurl>/_Layouts/AreaTemplateSettings.aspx Take into account that there is no way to get to this link via UI. For site collection you are using Site Sections –>”Page layouts and site templates “ under “Look and Feel”, but for Site you need to use hardcoded URL
3) Pick the templates you want to be available for this site