When I flip through a thesaurus (sometimes on the couch with a mug of tea, sometimes distracted on the train), 'whimper' usually branches into two main synonym directions: the soft, plaintive cry and the tone of weak, complaining speech. Common synonyms listed are 'whine', 'mewl', 'sob', 'snivel', 'moan', 'groan', and for animals 'yelp' or 'bleat'. A thesaurus will often cluster these by sense — so you'll see emotional/physical pain words like 'sob' and 'moan' near 'whimper', and more complaint-focused words like 'whine' and 'snivel' in another group.
What I like is how the thesaurus nudges you to pick the right flavor: use 'mewl' or 'yelp' for a childish or animal sound, 'snivel' when there's that self-pity element, 'moan' or 'sob' for deeper pain, and 'whine' when it's really a vocal complaint. Examples help: "The puppy whimpered under the porch" feels different from "She whined about the schedule." That little nudge is why I always consult a thesaurus: to catch the vibe, not just swap words mechanically.