Simple Malapropism or Onomatopoeia?

Last weekend I went to my cousin's birthday party. There were quite a few young children running around. After food and cake was eaten, two little boys grabbed a couple badminton rackets and began a volley with a little hollow plastic ball. They had been at it for a while when a younger boy got in their way. One of the boys shouted at him, "Get out of our way! We're playing tennis whack-it!"