Cook sushi rice according to package directions. Do not add vinegar or sugar like we would when making sushi. We just want plain rice.
Once cooled, dip your hands in the bowl of salted water and take a handful of rice. Mold it into a ball, triangle, or pack it (very tightly) into a cookie cutter, then unmold the shape. Or make a flat disc and put some leftover meat into the center. Fold up the edges of the rice disc to hide the filling, or add more rice on top and form into a ball or triangle. Keep your hands wet to keep the rice from sticking.
Using cookie cutters or a paring knife, cut fruits, veggies, meats, cheeses, sandwiches, or anything else you can think of, into cute shapes. Stack them, line them up, or create scenes and characters with them.
Silicone muffin cups are great for keeping foods separate that will either leak juice onto dry foods, or roll around like blueberries and peas.
Use craft punches or scissors to make shapes out of the nori (seaweed sheets) and stick it on the onigiri. It'll stick nicely to the rice.
For a great demo on making bento box lunches for kids, check out the alphamom website here: /family-fun/how-to-make-a-kids-bento-box/.