> in such case, the problem is how to return the upgraded socket object from child to main one?

Since SSL is a user-space layer on top of the kernel TCP socket there is no sane way to upgrade the socket to SSL in a child and then continue with the upgraded socket in the parent process.
You might try with threads but the OpenSSL FAQ clearly states that a single SSL connection should not be used from within multiple threads at the same time. I'm not sure if this applies in this case too where you are using different SSL connections but the same SSL context object.