Here is the full list of all 50 Pixel Puzzles Junior achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Complete Puzzle Rainbow
Complete Puzzle Roach
Complete Puzzle Cows
Complete Puzzle Bunny
Complete Puzzle Beach
Complete Puzzle Blur
Complete Puzzle Frog
Complete Puzzle Western
Complete Puzzle Girl
Complete Puzzle Dragon
Complete Puzzle Mirror
Complete Puzzle Owl
Complete Puzzle Monster
Complete Puzzle Cavemen
Complete Puzzle Racoon
Complete Puzzle Egg
Complete Puzzle Snowman
Complete Puzzle Boat
Complete Puzzle Sealion
Complete Puzzle Sunny
Complete Puzzle Ladybugs
Complete Puzzle Witch
Complete Puzzle Cave
Complete Puzzle Hiss
Complete Puzzle Smile
Complete Puzzle Whale
Complete Puzzle Fish
Complete Puzzle Jester
Complete Puzzle Broom
Complete Puzzle Snake
Complete Puzzle Veggies
Complete Puzzle Castle
Complete Puzzle Elephants
Complete Puzzle Night
Complete Puzzle Wolf
Complete Puzzle Wrestler
Complete Puzzle Cake
Complete Puzzle Octopus
Complete Puzzle Kangaroo
Complete Puzzle Blobs
Complete Puzzle Dino
Complete Puzzle Alien
Complete Puzzle Turtle
Complete Puzzle Sun
Complete Puzzle Lion
Complete Puzzle Flower
Complete Puzzle Reindeer
Complete Puzzle Birds
Complete Puzzle Colors
Complete Puzzle Fire