This sentence contains the word むにゅむにゅ, its a word that doesn't have an entry on jisho.org and also doesn't seem to appear on any of the extensive lists of Japanese onomatopoeia words. I eventually managed to find a page describing it as "gummy", but I'm not exactly sure how that relates to this sentence and ぐりぐり as the two words seemed opposed. One being hard and other soft. Could anyone help me out with how you might translate this clause? And what exactly むにゅむにゅ means?