Magical Boomerang Kinstones
Find the four Tingle Siblings and fuse Kinstones with them
Magical Boomerang Kinstones is a side-quest in The Minish Cap. It involves seeking out Tingle and his three brothers and fusing kinstones with them. Doing so will uncover an entrance in four trees in North Hyrule Field, which in turn reveals the final location of the Magical Boomerang.
To find Tingle, head into South Hyrule Field. Near the exit into the Eastern Hills is where Tingle is located. He's on a ledge where there's a hole nearby. Use the Cane of Pacci to hop up there and fuse with Tingle to open the top-left tree in North Hyrule Field. Head inside where you'll find a chest with a Red Kinstone piece and a switch. Step on the switch to light the first torch.
Using the Cane of Pacci to reach Tingle
To find Ankle, head inside the coral in Lon Lon Ranch and towards the north-west corner where you'll find a cave. Inside, split into two and push the large block out of the way. Climb the ladder and Ankle will be at the top. Fuse Kinstones with him to uncover the top-right tree. Inside the tree is another chest with another Red Kinstone and the second switch.
The cave to reach Ankle in Lon Lon Ranch
Using two Links to push a block in the Ankle cave
Knuckle is probably the trickiest to find, as he's got quite a few barriers between you and him. Head into the Trilby Highlands and, near the entrance to Mt. Crenel, head up the bridge and use the Mole Mitts on the soft wall. Dig with the Mole Mitts to where the ladder is and head up. Outside, Knuckle is waiting, so fuse with him to get the usual tree opening. Inside you'll get another Red Kinstone and the third switch.
Using the Mole Mitts to enter the Knuckle cave
David Jr. Fusion
To find David Jr., head into Lake Hylia from Lon Lon Ranch. Head around the path past Stockwell's House and you should see David Jr. immediately. Fuse kinstones for the bottom-right tree to be unblocked. Inside is yet another Red Kinstone and the final switch.
After pressing down all four switches and lighting all four torches, a ladder appears in the center of the four trees leading into the previously unreachable center. Climb down and open the large chest for the Magical Boomerang, which is quite the upgrade over the previous boomerang. The Magical Boomerang allows you to change its course in mid-flight by pressing the direction buttons in the direction you want it to fly.