Take the kids out on Lake Washington to catch crayfish.
The Washington Department of Fish and Wildlife has a website that lists what regulations need to be followed when you go crayfishing. On that site, they have a document that states how to identify the different types of crayfish species. You'll need to be able to identify what is what so that you'll know what the legal limit is for each type.
Be sure to buy the correct fishing permits before heading out.
You'll need a crayfish trap. They come in various sizes and prices. You should be able to find them for less than $20. Bass Pro Shops and Cabelas would be good places to start looking so that you can get an idea of what is out there.
Bring a cooler and some ice to get your catch home. Refer to the Food Safety section below.
Usually oily fish is the best bait. In this case, it could be fish heads or other parts of the fish that you would normally just throw away. Chicken will also work. Necks and legs could be used. Whatever is cheaper.
To make your bait last longer in the trap, put the bait into a plastic mesh bag or something like that. A mesh bag will restrict how much or how fast the captured crayfish can eat your bait. The longer your bait lasts in the trap, the more crayfish it will attract.
You'll need a boat of some sort to go out with. Yarrow Bay Marina rents motor boats by the hour. You could also think about using kayaks too.
The Leaf site has good advice about keeping crayfish alive. Basically you want to keep the crayfish cool, wet and out of water. If you put them in a bucket filled with water, they'll quickly use up all the oxygen in the water and die.
The best way to keep them alive would be to put them in a cooler with ice. Bring a wire rack that you can place inside of the cooler to keep the crayfish off the bottom. If the crayfish sit in the melted ice water, they might use up all the oxygen in the water, drown, and die.
Last Bit of Advice
Munchies has an interesting article about crayfish from this area. The Seattle Times has an article that relates directly to crayfishing on Lake Washington.
Field and Stream lists some different ways to go after crayfish. These approaches require less gear and should keep the kids entertained just as well.
Bring a camera to get pictures of the kids with their catch. They'll be proud to be able to bring home dinner.