Ok I too ran into that same error and didn't see why that was happening. Maybe that field is not available however its in the enum (its actually a struct but you get my point). There is always another way to skin the cat though. Once you have your references you could simple do this:
//using the method I posted above
var vApps = GetQueryvAppReferences("MyVAppName");
foreach (var vApp in vApps)
{
//get the reference to the vapp here
var vAppObject = Vapp.GetVappByReference(_client, vApp);
//now its easy to get the children vms
var vms = vAppObject.GetChildrenVms();
}
I find using the query service you have to write way more lines of code then you need to.