It is likely that you are not specifying the correct realm name. I assume that you have an application created and requested API access for your realm.
When making the request, make sure that the value of the realm query parameter is exactly the same as your realm, e.g. myrealm-child-T or myrealm-T. The value can be case sensitive, so check that the casing of the value is correct.
Where can you find your realm name?
It will be included in the URL when you login to the Ariba UI.