the key was the variable $object. Now this works as wanted!!!
children = GetObjectChildren($object);
foreach(p : children)
{
trace(1, "Child object: name='" . p->name . "' id=" . p->id);
interfaces = GetNodeInterfaces(p);
foreach(i : interfaces)
{
trace(1, "Interface: name='" . i->name . "' id=" . i->id);
SetInterfaceExpectedState(i, 2);
}
}