This is what you'll need (I used "swimsuit elastic" because it's a natural color that will match khaki pants, and it's super soft!)
measurements are for size 12 months to 2t, adjust length if you need :)
**It also works with 1" elastic-JoAnn's sells some colored elastic in the notions section
***JoAnn's is also where I purchased the vest buckles-in the notions section
****You can buy suspender clips at any fabric store, I bought my round ones on etsy.