Look up triboelectric series. See the list of materials. Choose clothing that is closer on the series to the material the slide is made out of. The farther apart the materials, the greater the degree of charging you're going to get. I.e., there will be a huge difference between wearing clothing made of cotton & clothing made of polyester.
If I were a smart ass, I would suggest covering the bottom of the slide with a sheet of aluminum foil. Then, have the kid wear something that makes a lot of static. Then, rolling some aluminum foil down the top of the slide, then touching the top with one hand and the bottom with the other hand. I'm just joking though - DON'T DO THIS - it will knock you on your ass (or worse.)
Edit: I changed my mind. Do this and post the video. Do *not* allow your kid to do this though.