Well we're probably hijacking the thread a bit here with a discussion of simulated messes. But if I can just leave you with one word... "cornstarch".
Okay, a sentence or two: Put about 1 cup of cornstarch in a soup pan and add about 3 cups of water and turn the heat up high. Stir constantly - I cannot over-stress this: keep stirring as the water heats up. For the first few minutes you'll think "this is stupid. nothing's happening. looks like milk". Then right before it starts to boil you'll see it magically begin to thicken. It usually happens quite quickly at this point. Reduce the heat to low and keep stirring and stirring. It's white and has zeo smell, so if you're looking for some realistic color add some canned pumpkin or marmalaide. But the first time through, just do it straight so you can get used to the sensation and water/cornstarch ration that's best for you.
Pour it onto a cookie sheet and allow it to cool. When it's cooled enough to pour down your pants, use a spatula (or your hands) to scrape it all up and place in a large plastic bag. When you're ready, simply pour into your diapers and enjoy.
It is an amazingly erotic sensation, and has a consistency that is extremely close to the real thing. Best news is that (a) it does not stain and ( is water-based, meaning it rinses down the tub with nothing but water. As I said, if you're playing with someone else and you want the humiliation of something that looks brown, then experiment with adding some banana (for smell), some mincemeat filling (sold in jars), and/or some really dark marmalaides for added stickiness and a sickening realistic color.
I swear...if you were a special effects director doing this for Hollywood, this would be your recipe. Once you use cornstarch, you will never turn back.
Diaper Dan