I hadn't seen that second vid, cool! That looks like a living creature. Man I wish I had a variable speed vibr.. um..
Seriously though, me and the kids do corn starch experiments from time to time. They're at the age that they like goopy crap anyway. Cornstarch is cool in that under pressure it gets really solid. Mix cornstarch and water in a baking pan/cassarole dish or big bowl, then poke your finger in it. Feels like pancake batter, very runny. Now smack it quickly and it's hard like a surface. If you put your fingers in it and pinch quickly, it'll feel like you're pinching a solid object, and as you release pressure, it gets runny again. Vary the cornstarch/water ratio till you get it just right. There's plenty of kids-science sites on the web, find one to get the basic recipe, then.. experiment! Good old fashioned science fun for cheap!