Short grain rice is the secret to making rice balls that stick together. The short grains are plump and slightly sticky. Don’t use long grain rice — it’s too fluffy and won’t hold together! Instead, everything will fall apart. There’s not enough starch in long grain rice to act as “glue.”
Also, heat to activate the starchy “glue” in short grain rice. Warm rice is much easier to roll into balls. And pack tightly with your hands!
If the rice balls are still falling apart, I suggest omitting the sesame oil completely and cutting the kimchi into very small 1/4-inch pieces.