How does getChild() and getChildCount() work?

edit

I am making a plugin for this model:

<?xml version="1.0"?> 
<sdf version="1.4">
  <model name="my_model">

    <link name="link">
      <gravity>true</gravity>
      <inertial>...</inertial>

      <sensor name="camera" type="camera">
        <pose>0 0 0 0 0 0</pose>
        <camera>
          <horizontal_fov>2.0</horizontal_fov>
          <image>
            <width>640</width>
            <height>480</height>
          </image>
          <clip>
            <near>1.0</near>
            <far>15000</far>
          </clip>
        </camera>
        <always_on>1</always_on>
        <update_rate>10</update_rate>
        <visualize>1</visualize>
      </sensor>

      <collision name="collision">...</collision>
      <visual name="visual">...</visual>
    </link>

    <plugin name="myplugin" filename="libmyplugin.so"/>
  </model>        
</sdf>

In the plugin I want to manipulate the pose of the camera. The code I use to do this is:

gazebo::physics::BasePtr base = _model->GetLink()->GetChild(std::string("camera"));
if(base){
    boost::dynamic_pointer_cast<gazebo::sensors::CameraSensor>(base)->SetPose(pose);
}

Interestingly base always ends up being a nullptr.

When I call

_model->GetLink()->GetChildCount()

I get the children count of 1.

If I call

_model->GetLink()->GetChild(0)->GetName()

I get "collision". So it seems like the only children to the <link> is the <collision>. This seems quite unlogical for me and I am stuck. I also tried adding another collision tag to the model SDF, GetChildCount then increases to 2, and both collisions can be listed. Is there something about the semantics of GetChild() that I do not understand? I thought I could manipulate the camera using it?