You’re all doing it wrong! There’s two ways to do it right. Either make your own variable to replace HEADER_IMAGE_HEIGHT and HEADER_IMAGE_WIDTH in your functions.php and define a new variable (also replacing the variables in your template files), or remove the filter and redefine it in your functions.php:
remove_filter( 'HEADER_IMAGE_WIDTH', 'twentyeleven_header_image_width' );
remove_filter( 'HEADER_IMAGE_HEIGHT', 'twentyeleven_header_image_height' );
define( 'HEADER_IMAGE_WIDTH', apply_filters( 'child_header_image_width', 1280 ) );
define( 'HEADER_IMAGE_HEIGHT', apply_filters( 'child_header_image_height', 200 ) );
I prefer the latter so I don’t have to find/replace HEADER_IMAGE_WIDTH & HEADER_IMAGE_HEIGHT with new variables. I just add those four lines to my functions.php and I’m done! Since it is in my child theme, I never have to worry about it getting overwritten on update and if you don’t remove_filter() on each first, it will show in your debug log (if you use one) because those variables are already defined.