Have you ticked Enable Auto-Align for the dropzone?
I believe when you have that ticked the elements will auto-align to the left like your example. I think there is not an auto-align to the center/middle so you could leave the enable auto-align unticked. Then the images would just be anywhere in the dropzone where the student would have placed them. This probably is not the solution you are hoping to use with elementary students.So when we create drag and drop activities with images, we try to make the images all the same size so that each object fits perfectly in the dropzone. So all images would have the same size transparent background that would match the same size of the dropzone. Then we add the images circle, square, rectangles, etc... to be placed in the center of the transparent background and save each image. This way we can enable auto-align for those elementary students and it works the way you want.