Is it possible to design a slide that would allow a child to get enough momentum going down to be able to do a loop without tumbling out of the slide? I know this would be terribly dangerous, but it would also be pretty awesome if it worked. I know some skateboarder (Tony Hawk?) went through a loop on a skateboard, which suggests to me that it is feasable.
Well, let’s see. Suppose for a moment that we somehow found a frictionless slide, and we made a generous circular loop (although a clothoid loop would be better) with a diameter of 2 meters, enough for a kid to get through without getting too crunched up. In order to make it around the loop (just staying in contact), the child would have to have a speed at the top of the loop equal to:
sqrt(radius * g) = sqrt (9.8) = 3.1m/s
As she enters the bottom of the loop, the child would need be traveling at 7m/s (from energy conservation considerations) to ensure a 3.1m/s speed at the top. This translates into a slide that is 2.5 meters tall, or about 8.2 feet up. That’s a pretty tall slide, but not unheard of. Another issue (of many…) to consider would be the centripetal acceleration at the bottom, which comes out to 5g’s, which would not be horrible but probably unpleasant.
With friction, all bets are off, though, and you’d probably want a slide twice that tall and you’d also want a sign that said “make sure you wear slippery pants.” This would compound the g forces, and if you were going twice as fast at the bottom of the slide, you’d end up with a centripetal acceleration of 20g’s which would most certainly knock you out, or at least throw you for a loop. 
It would probably be better to attempt to build one at a water park, as you can assume low friction and fast speeds. You’d want to make the radius of the loop large enough to minimize the painful acceleration, but it seems plausible to imagine a loop in a waterpark slide if you made it high enough and with a big enough radius. But, the bigger the radius, the longer the fall if you don’t go fast enough, and the last thing you want is kids falling from the top of a 4-meter tall loop onto a hard plastic slide below. I suppose you could make it so that anyone who wasn’t going fast enough landed into the water (just make the slide corkscrew a bit so the top is over the water and not the bottom of the slide). It would certainly be fun to watch on the first and only day the park is open.
-Tofer
Problems, problems.
Aside from the friction problems, human children tend to flop and flail about, causing their center of gravity to go hither and yon. It would be a bit like trying to get a half-filled water balloon to loop on a hotwheels racetrack.
There was a fully-enclosed, looping water slide at the now-defunct Action Park. (a.k.a. Traction Park.)
I used to go there every summer as a kid and nearly kill myself on the Alpine Slide of Death or Minor Fiberglass Burns.
Ah, memories.
Possible.
Get the parents to sign a waiver first, though.
Also consider that the kid’s head is going to be near the center of the loop, and will therefore be subject to less gees. Unless e loses es balance, and falls over backwards, which I’m sure would never happen.
A simpler and safer (but still thrilling) option might be a more horizontal loop, like an extreme version of the corkscrew slides you see sometimes. You could either enclose it for safety, or put it over a safe landing like a pool. I don’t think it’d be too hard to put a slight inversion in, so that local “up” would be mostly sideways, but a little below horizontal.
I think too that the kid would have to be lying down to make it through the loop.
No way with that many Gs that your going to keep yourself in an upright position.
A picture of it can be seen here
It looks really horrible; if you fail to go around the loop, you’re basically stuck in there. Also, what prevents the water flow from stalling and accumulating in the bottom of the bend?