Today's kid's churches - at least the one I help run - tends to be quite media based, so when I have a session, I like to try and avoid that and use craft or an object lesson instead. As one experienced kid's worker pointed out, the lack of media can actually hold their attention longer because something as old as a flanograph or as simple object lesson may be something entirely new for them.
I based my object lesson on Genesis 3. Like sin in general, the fruit on that tree looked good on the outside, but it wasn't so great once Adam and Eve bit into it. If sin looked horrible and 'wormy' then we wouldn't be tempted, but satan is smarter than that. That's why we need to be on guard and look for marks on the outside and compare it to the Word of God.
That essentially sums up the lesson I gave my girls. But before we started, I told them that we were going to have apples to snack on. Upon cutting open the first one, the girls were disgusted. The inside was black and looked revolting. They had several shudders before proceeding to demolish the good apple.
I think they failed to notice that I was neither surprised, nor disappointed. In fact, I was having a hard time containing myself, because it worked exactly as I planned. Here's a picture:
No, that is not mould. It is not 'off stuff'. It won't even taste bad or make you sick if you eat it. It's food dye. After google had no answers on how to infect an apple from the inside, I had to get creative myself. If you want to use this object lesson for your kid's group, here's how I did it. (And if you don't want to use it you can either stop reading here or read it anyway :P)
Get the longest sewing needle you can and some thread. (I used 1 ply crochet cotton that is too thick to put through the sewing machine and too thin to crochet with. ;) ) Thread the needle and push it down the core of the apple from the top so that the thread runs from top to bottom through the middle of the apple. There should be about two inches of thread dangling from the bottom of the apple. Repeat a few times coming in from different angles, but trying to cross through the core of the apple as many times as possible. (I think I had four threads in my apple). In a glass, pour in a tablespoon or two of black food dye, available from supermarkets. Sit the apple in the glass. The apple should not be touching the food dye, but the threads should be sitting in it. Essentially what happens is that the food dye travels up the threads and leaks into the flesh of the apple. This will take about 24 hours. When the dye is finished leaking just wash the apple under the tap. You will want to wrap the apple up in a cloth or something and travel with it in a zip-lock bag or something like that, because the dye does like to go places it shouldn't. :P