Lalaloopsy/WMG

The Lalaloopsys were once humans, but died in their childhood.

Think about this. The Lalaloopsys each moved into the same neighborhood in their human lives at different points in time. The neighborhood was cursed by a witch who hated bad behavior in children, and so she killed each of the children who had done wrong and their innocent siblings. They were then reincarnated as colorful rag dolls who are perpetually happy, that is, until they find the secret. Here are some examples of the bad deeds, ranging from minor to major in random order:

Pillow Featherbed had slept in class. Her younger sister, Blanket had always kept Pillow up every night with her incessant laughing. They both died from a carbon monoxide leak in their house.
Crumbs Sugar Cookie never ate her vegetables. She got diabetes and then died of pancreatic cancer. Her sister, Sprinkle was killed by her parents before they killed themselves out of grief.
Mittens Fluff 'n' Stuff and her sister Bundles didn't listen to their mother when they were told to come inside because of snow. They went missing and were found a few days later dead in the snow from hypothermia.
Peanut Big Top brought her sister, Squirt to run away to the circus. They caught swine flu from one of the boars in the circus acts and died.
Spot Splatter Splash did graffiti in the urban area of her town. She was then beaten to death by gang members. They went to her house after hiding her body and then killed her family.
Jewel Sparkles thought more highly of herself than of anyone else she knew. She and her sister Trinket died of sadness after getting coal for Christmas.
Dot Starlight made nuclear weapons. One of her weapons malfunctioned and killed her.
Bea Spells-A-Lot cheated on a test. She was in a class trip accident with her younger sister and her parents.
Blossom Flowerpot and her sister Petal picked flowers from another person's garden. She died from contact with a poisonous flower.
Tippy Tumblelina danced for other people. She didn't really know that dancing for everyone was inappropriate. A child predator was so tempted by her, and that led to him raping and killing her. Her little sister, Twisty, committed suicide.
Sunny Side Up and Berry Jars n Jam grew marijuana. One of their friends ratted them out to the police, and thinking that everything was over for them, they both committed suicide.
Patch Treasurechest was a little boy on a pirate ship that landed in the cursed neighborhood. When the pirates found the buried treasure, he got greedy, and wanted the treasure for himself. The pirates killed him out of rage.
Charlotte Charades and her little sister Sherri were street mimes. They were killed personally.
Ember Flicker Flame and her little sister Red both tried to rescue a person from a burning building. They all died in a fire.
Prairie Dusty Trails and her sister Trouble both fended off outlaws using guns. They were brutally shot by them.
Marina Anchors and her brother Matey went on a cruise trip. They died from oxygen deprivation while the ship crashed.
Ace Fender Bender tried to drive away in his dad's car. I think it's safe to say he crashed.
Forest Evergreen carved his initials into trees. One unsteady one fell onto him.
April Sunsplash didn't listen to her mother while she was out in the rain. She later died of pneumonia.
Mango Tiki Wiki and her sister Kiwi were pineapple smugglers. They were killed by the headhunters.
Confetti Carnivale crashed into a private masquerade party. She was later beaten to death by angry party guests.
Harmony B. Sharp does nothing more than trying to outclass all the musicians and bands. She was later killed in a fatal tour bus accident.

Other deaths are open to interpretation, although I sure do have other ideas.