After today's trial, I think I'm going to have to reevaluate. The recipe was super simple (just flour and baby oil [and food coloring, if that's your thing]), the boys enjoyed "dumping" all the ingredients, and they had a total blast playing with it. That doesn't seem surprising unless you consider that neither of my boys really prefer to get their hands dirty (I have no idea where that comes from). However, it was messier than I think I can justify for an indoor birthday party. I'm not giving up on the idea...just brainstorming other options/tweaks.
Here's the recipe I used for it, although I did a half-batch and ended up using a lot more baby oil to reach the consistency I wanted.