I would model it like this:

public class Person
{
    public String Name { get; set; }
    public String MiddleName { get; set; }
    public String LastName { get; set; }
    public DateTime Birthday { get; set; }
    public BirthCertificate BirthCertificate { get;set; }
    public MarriageCertificate MarriageCertificate { get;set; }
    // ...etc...
}

public class Certificate
{
    public String Code { get;set; }
    public DateTime IssueDate { get;set; }
    // ...etc...
}

public class BirthCertificate: Certificate
{
    public DateTime BirthDate { get;set; }
    // ...etc...
}

public class MarriageCertificate: Certificate
{
    public String SpouseName { get;set; } // or Spouse could also be a person
    // ...etc...
}

public class Employee
{
    public ISet<Person> Children { get; }
    // ...etc...
}

Some points:

• Note the ? usage which means certificates are optional.
• Certificate deserve their own types. If you have more than one property that start with the same prefix, most of the time, it means you can define an object off them. I have also created a base Certificate class because they may share some common properties and behavior.
• Children is a collection of Person objects.
• Spouse could also be a person, if you will (the property would then be named Spouse).
• I don't repeat the declaring type name in a property name: Name instead of PersonName