You can define your "orders" as just complex type and refer it to the element in your main object, so something like :
<xsd:element name="Orders" type="xsd:Orders"/>
More info here:
Thanks for the help, the reference you posted was really useful. In an extension to this, is there a way to have multiple level children - I need to make one that is 4 layers deep.
In Mendix it is not possible to map a recursive structure. If you know that you will have a maximum of 4 levels, then you have to manually map them. The drawback of course is that if you decide to change something, you have to change the mapping in al 4 layers.