For Mico you have to use the poa hierarchy in the corbalo url as well. For your example corbaloc::localhost:12345/childPOA/hello
should work.
See also the Diploma thesis of Frank Pilhofer, the implementor of Mico's POA, which states
create a persistent POA with the name ``MyPOA'', and then activate an object
using the ``MyObject'' Object Id, you could refer to that object using the IOR
iioploc://thishost:1234/MyService/MyPOA/MyObject
Edit: You have to start your serving executable with a service name -POAImplName MyService