This is completed in the chapter Rose's Room - Hopscotch. You will pass some numbered blocks and an etch-a-sketch, that you can actually use! After jumping across all the blocks, you will notice a piggy bank with some coins around it. Put all the coins inside of it and then slam down on it to break it. You'll need to reload the checkpoint so your coop partner can do the same.
Here is a full walkthrough of the game with all achievements / trophies and minigame locations, including timestamps below!
Timestamps:
0:03 Minigame 1/25 - Whack A Cody (The Shed - The Depths)
0:28 Minigame 2/25 - Flip The Switch (The Shed - The Depths)
0:59 Fried Friendship Achievement / Trophy (The Tree - Captured)
2:05 Minigame 3/25 - Tug Of War (The Tree - Captured)
2:36 Struck A Pose Achievement / Trophy (The Tree - Deeply Rooted)
3:12 Minigame 4/25 - Plunger Dunger (The Tree - Deeply Rooted)
3:38 Minigame 5/25 - Tank Brothers (Rose's Room - Pillow Fort)
4:04 Plastic Prison Break Achievement / Trophy (Rose's Room - Pillow Fort)
5:30 Minigame 6/25 - Spacewalk (Rose's Room - Spaced Out)
6:15 Minigame 7/25 - Laser Tennis (Rose's Room - Spaced Out)
6:50 Faraway Frequencies Achievement / Trophy (Rose's Room - Spaced Out)
8:26 Look At Him Go Achievement / Trophy (Rose's Room - Spaced Out)
9:08 A Daring Devil Achievement / Trophy (Rose's Room - Hopscotch)
9:42 Minigame 8/25 - Rodeo (Rose's Room - Hopscotch)
10:00 Minigame 9/25 - Feed The Reptile (Rose's Room - Hopscotch)
10:27 Breaking The Bank Achievement / Trophy (Rose's Room - Hopscotch)11:09 Minigame 10/25 - Batting Team (Rose's Room - Hopscotch)
11:30 Snackosaurus Achievement / Trophy (Rose's Room - Dino Land)
12:16 Minigame 11/25 - Birdstar (Rose's Room - Once Upon A Time)
12:48 On The Rails Achievement / Trophy (Rose's Room - Once Upon A Time)
13:35 Realize Your Art Achievement / Trophy (Rose's Room - Once Upon A Time)
14:29 Minigame 12/25 - Bomb Run (Cuckoo Clock - Gates of Time)
15:09 Platforming Prodigy Achievement / Trophy (Cuckoo Clock - Gates of Time)
15:54 Minigame 13/25 - Horse Derby (Cuckoo Clock - Gates of Time)
16:16 Force Triangulated Achievement / Trophy (Cuckoo Clock - Gates of Time)
17:39 Lost And Found Achievement / Trophy (Snow Globe - Winter Village)
19:12 Mood Swing Achievement / Trophy (Snow Globe - Winter Village)
20:02 Minigame 14/25 - Icicle Throwing (Snow Globe - Winter Village)
20:35 Minigame 15/25 - Snow Warfare (Snow Globe - Winter Village)
21:27 Minigame 16/25 - Shuffle Board (Snow Globe - Winter Village)
22:35 Something Fishy Achievement / Trophy (Snow Globe - Beneath the Ice)
23:03 Terror of the Seven Seas Achievement / Trophy (Snow Globe - Beneath the Ice)
23:50 Minigame 17/25 - Ice Race (Snow Globe - Beneath the Ice)
24:31 Bug Sized Relaxation Achievement / Trophy (Garden - Green Fingers)
25:10 Minigame 18/25 - Garden Swings (Garden - Weed Whacking)
25:39 Minigame 19/25 - Larvae Basket (Garden - Weed Whacking)
26:13 Minigame 20/25 - Snail Race (Garden - Frog Pond)
26:35 Minigame 21/25 - Slotcars (The Attic - Setting the Stage)
26:56 Minigame 22/25 - Chess (The Attic - Setting the Stage)
27:08 Minigame 23/25 - Musical Chairs (The Attic - Setting the Stage)
27:37 Minigame 24/25 - Volleyball (The Attic - Symphony)
27:58 Meditation Maestro Achievement / Trophy (The Attic - Symphony)
28:28 Minigame 25/25 - Track Runner (The Attic - Setting the Stage)