This project is clever in a few ways. First is, in purchasing an organic New Twist mattress + bed frame kit, this couple also created a side car crib by simply not trimming down the boards on one side and adding the center support kit below. They also used a combination of 2X4's and 2X2's instead of 2X3's because their Home Depot did not have them in stock.
When their little one is ready for her own room, they can cut down the side car crib boards or simply shift them to the center to make side tables. Very cool project and I am so happy that they shared their design and progress photos with us!