Once you make a batch, the onigiri can easily be frozen or kept in the frig. You can eat them cold or warmed or grilled. You can fill them with almost anything. See? Perfect. Seriously-what kid doesn't like rice?? (Um, confession. My four year old won't eat rice. Really.)
Once I discovered Just Bento, I found the best news. It is an entire site dedicated to easy to make dishes that are healthy, tasty, and can be stored at room temperature. In other words, they can be pulled together quickly, can stay in a lunch box or kept in a bag for day trips.