sand clay

1 cup sifted beach sand (or sand from sandbox)
1/2 tsp alum (look at a pharmacy)
1/4 cup corn starch
1 cup water

Your kids can help gather the sand and measure and stir ahead of time, then you should take over. Heat on a medium burner and stir it a lot until it gets thick. (Use an old pan for this.) Let it cool enough to use. You can store it in a sealed container pretty well for an hour or so if yu want to prepare it in advance. Spoon some out on a paper plate or a regular plate covered with foil so it doesn't stick. Your children can mold it into a paper weight or castle or whatever you want. It doesn't actually hold a shape as well as real wet sand so you can't get too detailed with your structure. You can press shells, rocks, other beachy stuff in (put glue under them. Just apply it right on top of the wet sand clay to make sure the stuff sticks well.) or shape it more like a mounded castle and make little toothpick flags to put on top. I saw an idea on thriftyfun.com to cover a picture frame with the sand clay and then press shells and stuff in to make a nice beachy frame. I have also thought about filling lots of ice cube trays with the clay and when it dried you would have these nice uniform sand building blocks to glue together to make a larger structure. It doesn't dry completely for a few days so try to leave your creation out of the way until your children can handle it with no risks. Once your sand clay is dry it is very strong.