Hi,
I'm brand new to chickens though I've raised a lot of other baby animals...one of the best things I've ever found for under them is the incontinence pads they use on hospital beds. Not the disposable ones, but the washable kind. They're pretty waterproof (unless they stay wet a long time, then they will soak through), they absorb liquid and pull it away from the babies, they're not slippery, and they are made to be washed in hot water with lots of bleach and dried in a hot drier.
My chicks are two weeks old and I just put litter in their brooder, but I think I'm going to take it out - the pads are SO much easier to clean and also to see the poops. And the litter keeps getting scratched into the waterer. Ick.
The only thing I've had to watch is that I use pads in good condition so there aren't strings for the chicks to eat or loose stitching to get toes caught in. I've used them for years for litters of puppies, for sick adult dogs, in cat carriers on vet trips, etc. They are excellent.
For the chicks, I put several layers of newspapers down in the brooder first, then one large incontinence pad, and now that they're big enough to get on top of the feeder and waterer, I laid a baby gate across the top of the brooder like a roof so no danger of them flying out. I'm new to chicks, so if any of this sounds dumb, please tell me - but so far it's worked very well.