[KnownType(typeof(SuperClass))]
[DataContract]
public class BaseClass
{
...
}

[DataContract]
public class BaseClass
{
    [DataMember]
    public string Name { get; set; }
}

[Serializable]
public class ChildClass: BaseClass
{
    public string SecondName { get; set; }
    public int Age { get;set; }
}

<ChildClass xmlns="http://schemas.datacontract.org/2004/07/Serialization" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
  <Name>Ivan</Name>
  <_x003C_Age_x003E_k__BackingField>1</_x003C_Age_x003E_k__BackingField>
  <_x003C_SecondName_x003E_k__BackingField>Petrov</_x003C_SecondName_x003E_k__BackingField>
</ChildClass>

[DataContract]
public class BaseClass
{
    [DataMember]
    public string Name { get; set; }
    [DataMember]
    public string SecondName { get; set; }

}

[Serializable]
public class ChildClass: BaseClass
{

    public int Age { get;set; }
}

<ChildClass xmlns="http://schemas.datacontract.org/2004/07/Serialization" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
  <Name>Ivan</Name>
  <SecondName>Petrov</SecondName>
  <_x003C_Age_x003E_k__BackingField>1</_x003C_Age_x003E_k__BackingField>
</ChildClass>