While I love Alley Cat Allies, I'm not in love with everything they say about socializing feral cats and kittens. I would not take their advice to separate the kittens.
There are some other things you can do. Instead of just putting out the food for them, you can take an old t-shirt or sweatshirt, sweat it up REALLY good, and place it under the food dishes. If you've got more to spare, put one where they sleep. This will also help them come to associate you (your smell) with good things.
As to the regressing - this is normal. It's two-steps forward, one-step back (or one step forward, two-steps back
) when socializing ferals.
The most important thing, really, is to let them be on their own schedule. For all intents and purposes, they are wild animals, and patience is the key. Remember - they don't know they like being loved. They don't know they like being petted. They don't know a brush feels good. They don't know toys are for playing with. They don't know they've got a purr motor that can be turned on and off with human interaction. And the only way they get there is learning to trust you. Each animal has different thresholds for that "trust barrier" - and none of them are on any kind of human time frame.
As to play - they don't know what play is either. However, they're kittens! There should always be some toys out for them to play with. We found the toys would often be completely relocated between when we went to sleep and when we got up.
They figure it out.
But using wand toys to interact with them can help them learn play - both get used to you and to put you in a "not harmful" category. With us, we had every wand toy there is. But the best was one we made ourselves. We purchased those really long thick (kind of square) leather shoe laces (used to lace up boots or something), tied a knot in one end and taped the other to a dowl rod. We would wiggle it (the knot) slowly across the floor. Kitt(ies) would inevitably pounce! It is just too enticingly like a mouse-tail. Some loved the "under the rug" game the best. The minute that "mouse tail" went underneath the throw rug they were digging furiously, lol!
Just make certain you put all wand toys out of any kitty reach when you're done for the day. They do tend to eat them (the string, literally), and it can require surgery to fix. Thankfully we didn't have to learn that the really hard way - just the hard way. I did have to examine one cat's duty for about two days to make sure that string came out!
Music does seem to help. Tuning a radio to a classical station and leaving it on while you're out or at work can help them feel more comfortable.
And - just being in the room they're in - but not paying attention to them. Doing something else - reading, writing, sewing, whatever. Even reading out loud. Letting them see that you're just there - not focused on them. Just - not being threatening.
Also, if you ever reach out to them for any reason, always do it palm down. This, too, is less threatening.
And as Vik61 pointed out, "not" looking at them can really help boost the trust factor. Looking at their forehead or over their heads - even closing your eyes for a few while you're looking toward them (just make sure that when you open your eyes you're not looking directly into their eyes). As Vik described, the slow blink is a great one. Looking into their eyes is a sign of aggression - so it's probably no coincidence that Vik61's kitty doesn't try to climb towards the window when she's around.
Mostly, just thank you for rescuing these kitties!
Are you going to keep them? Or are you going to adopt them out? Have they seen a vet? They probably ought to - they most likely need some type of de-worming medication, and products that are not prescribed by a vet don't work and can create other problems.
Just so you have the info, it is safe to spay/neuter them once they're 7 weeks old.
If you need to see if there are low-cost spay/neuter services, the link in my signature line will take you to a number of links where you can search to see if you can find anything in your area.