All child elements in fact are properties. To indicate which property accepts the child element as its value you must specify the ContentProperty attribute of the class.
C#
[ContentProperty("PropertyName")]
public class SomeClass
{
...
public PropertyType PropertyName {get;set;}
...
}
XAML
<SomeClass x:Name="someClassInstance">
Value
</SomeClass>
This way the last declaration corresponds to the following C# code
C#
someClassInstance.PropertyName = Value;
That's it!