Hi,
Generally once a site (hostname based) uses SSL I would split it off into its own installation.
But you can try this and it may work ok for just one site to have SSL:
<add key="SSLIsAvailable" value="false" />
<add key="Site2-SSLIsAvailable" value="true" />
replace the 2 with the correct site id for the child site that matches the certificate.
The above solution may work but it would only work for one site since you can only have 1 SSL certificate bound to the IIS site. It would not work for 2 child sites with different host names.
Of course for folder based sites the same SSL certificate can work since they all have the same domain in that case.
Hope that helps,
Joe