Congrats on the new slings. There are two things you can try that have worked successfully for me with feeding the tiny ones.
1. Try ripping off the hind legs of large crickets. I've found slings will eat them readily
2. Get the smallest crickets you can find and pre kill them. Just rip off the head or something and throw it in. The slings will eat them. There will probably be alot of leftovers though. Make sure to clean them out quickly as in a high humidity sling environment, they'll mold quickly.
As for the T-shirt idea, it sounds very interesting. I've never heard that one before. I would however worry about them chewing through it once they get a little bigger. I have told some people to use a wet towel across enclosure tops to help increase humidity in large tanks, your T-shirt idea is just smaller scale. Good for you
If the lizard litter is what I'm thinking it is, that stuff is horrible. Looks like little white/greyish pellets right? Gets all mushy and nasty if it gets wet. It might work with a terrestrial with no moisture, but I wouldn't use it anyway. Any time you dropped some water in there, it would get unusable. It's just as bad as that corn cob bedding crap.