title: ROS2: How to pass arguments from one launch file to a child launch file
body: <p>I have a main <code>bringup.launch.py</code> launch file of which the launch descriptor includes <code>child.launch.py</code> as a <em>child</em> launch file like this:</p>
  
  <pre class="lang-py prettyprint-override"><code>from ament_index_python.packages import get_package_share_directory
  from launch import LaunchDescription
  from launch.actions import IncludeLaunchDescription
  from launch.launch_description_sources import PythonLaunchDescriptionSource
  
  def generate_launch_description():
      package_prefix = get_package_share_directory('child_package')
      argument_for_child = "lala"
  
      return LaunchDescription([
          # include the child launch file
          IncludeLaunchDescription(
              PythonLaunchDescriptionSource([package_prefix, '/launch/child.launch.py'])
          ),
      ])
  </code></pre>
  
  <p>How do I pass an argument from <code>bringup.launch.py</code> to <code>child.launch.py</code>?</p>