I don't use a stone-washing process to soften linen fabric for sustainable reasons. Indeed, the process usually involves using high-dose chemicals, which may be harmful to the environment and workers.
Except for the linen fabric blends such as linen-viscose, linen-silk (those are soft and flowy enough already!), or garments I managed to take care of softening at home, your Kidphilosophy 100% linen garments demand some patience.
So you can wait for it to soften after many washes, or you can follow the simple recommendations below to "break the linen" to make it softer.
1. Start the first laundry at 40°C, then toss it in the dryer on low, or drip-drying it on a line. In the very beginning, wash a little bit more often.
2. For extra softness: soak the linen overnight in a solution of 1 cup of white vinegar to 1 gallon of water before washing.
3. Chemical (traditional) fabric softeners may not work well on linen fabric or work slowly; plus, they may add a chemical residue or a scent that you don't want. Opt for a natural fabric softening method by adding a solution of 1/2 cup of baking soda.
Or, more preferably, just be patient and enjoy this "story" of your garment. With time you will notice it softens and feels better against the skin. You will love it even more!