tl;dr: tricking children is easy, but not as rewarding as it sounds.
My older cousin convinced me the universe was heliocentric sometime near the end of the 2st grade. I knew about the solar system, and even knew that the planets orbited the sun, but I hadn't given any thought at all to what the stars actually were, and it made sense that the planets would be circling around something stationary. I believed it all through the summer, but it didn't actually come up until my 3rd grade science teacher told us about long-discredited historical theories like spontaneous generation, geo/heliocentrism, and the four humours. Thankfully I was aware enough to not argue with him about it, but I certainly felt foolish.
On the other side of the coin, my younger cousin was about that same age when he came up and excitedly told me about potato batteries. He didn't actually know what they were, just that they existed, and it was pretty easy to convince him that plugging a desk fan into a potato would power the thing. It didn't work, obviously, and told his older brother that he needed help. He followed him into the room, immediately put two and two together when the saw the potatoes with 3-pronged holes poked into them (he was in on the joke), and busted out laughing. He was a little upset about it and I felt kinda bad about it, and so did his brother. To help him save some face, and by way of an apology, we built an actual potato battery and got it to power a little light bulb.