Keep in mind that the word "sushi" refers to the rice, not the fish, therefore sushi can be anything as long as it's rolled in rice (maki sushi) or on top of a ball of rice.
Hopefully you have a sushi mat that helps with with the rolling. If not they're very cheap and can be purchased anywhere from asian markets to Bed Bath and Beyond. With the mat, all you need is a litte practice and it comes with detailed instructions.
Also, you might enjoy making a layered style of sushi which doesn't seem to be found so much in the states as it is in Japan. You start with a piece of Nori, press vinegared rice to cover the surface about 1/4" thick, lay on your next ingredient be it fish, raw or cooked, chicken or any combination you like, press on another 1/4" of the rice and another piece of nori and then slice with a very sharp knife into equal square pieces.
Many japanese chefs have special presses to make this sushi but I have yet to find one in the states.
There is also Chirashi sushi which is simply ingredients layered on top of a bowl of rice...no muss, no fuss.
Since you'll be having a party for kids, it seems kind of unfair to limit their creativity with recipes. Why not create an ingredient bar with lots of julliened vegetables, cooked shrimp, crab, smoked salmon, they won't go bad like actual raw fish will if left out) cooked chicken...heck, even crumbled ground beef and cheese (imagine, cheesburger sushi) and then they can come up with their own creations.
As far as side dishes, you can get frozen edamame in the shell and very quickly blanch them in boiling water and salt them. The kids might like picking the beans out of the pods. There are also frozen japanese dumplings (also called gyoza)that you can steam or fry. If you can't find the japanese ones, chinese pot stickers are essentially the same thing.
For dessert, there's green tea and/or red bean ice cream. They should have these at most asian markets but if you need to make them it's pretty simple:
Take about 4 tablespoons of green tea (open some tea bags if you can't find loose) and run it through a food processor or clean coffee grinder until it's a very fine powder. Add about 2 tablespoons very hot milk and stir and allow it to steep and come to room temperature.
Add the cooled mixture to 1/2 gallon of softened but not melted vanilla ice cream and fold it in as best you can a la Cold Stone Creamery (some of the sushi mats come with a paddle for the rice, this paddle is perfect for this application). Or if you have an ice cream maker, you can go ahead and melt the ice cream down, add the green tea mixture and churn it in the ice cream maker until it's firm again. This will not render a very pretty green ice cream so you may want to add a drop or two of food colouring. Most of the stuff you buy ready made has artificial colour in it anyway.
The red bean ice cream is a little more complicated. If you can't find a jar of the presweetened adzuki beans to mix in the ice cream, you'll have to make your own:
1 cup dry adzuki beans
1/3 cup white sugar
2 teaspoons lemon juice
3 1/2 cups water
In a saucepan, combine the azuki beans, 1/3 cup of sugar, lemon juice and water. Bring to a boil, and boil uncovered for 3 minutes. Reduce the heat to low, and simmer for 2 1/2 to 3 hours, or until the beans are very tender. When done, the beans and liquid should amount to 3 cups. If not, add more water to compensate.
You can strain the bean mixture through a sieve and discard the bean skins if you like, I leave mine a bit chunky. Refrigerate for about 2 hours, or until cold.
Add this to 1/2 gallon of vanilla ice cream or use it as a topping.