void positionChild(Object childId, Offset offset)

Specify the child's origin relative to this origin.

Call this from your performLayout function to position each child. If you do not call this for a child, its position will remain unchanged. Children initially have their position set to (0,0), i.e. the top left of the RenderCustomMultiChildLayoutBox.

Source 

void positionChild(Object childId, Offset offset) {
  final RenderBox child = _idToChild[childId];
  assert(() {
    if (child == null) {
      throw new FlutterError(
        'The $this custom multichild layout delegate tried to position out a non-existent child:\n'
        'There is no child with the id "$childId".'
      );
    }
    if (offset == null) {
      throw new FlutterError(
        'The $this custom multichild layout delegate provided a null position for the child with id "$childId".'
      );
    }
    return true;
  });
  final MultiChildLayoutParentData childParentData = child.parentData;
  childParentData.offset = offset;
}