mujoco

<mujoco model="child">
  <worldbody>
    <body name="body">
      <geom name="geom" size="1" pos="0 0 -2" rgba="1 0 0 1"/>
    </body>

    <frame name="frame">
      <body name="body2" pos="-1 0 0">
        <freejoint/>
        <geom name="geom2" type="box" size="1 1 1" rgba="0 1 0 1"/>
      </body>
      <body name="body3" pos="1 0 0">
        <freejoint/>
        <geom name="geom3" type="box" size="1 1 1" rgba="0 1 0 1"/>
      </body>
    </frame>
  </worldbody>
</mujoco>