There could be a number of things wrong, and you may not be able to save them. Sometimes they are just not meant to live. The boniness -- failure to thrive, or gain weight -- is not a good sign .I'll throw some ideas out there, though.
I'd have a brooder that is big enough that they can escape the heat or get under it. as they wish. The "right" temperature is so tricky, and not the same for all chicks.
Make sure your water is set up so it stays relatively free of litter so they always have plenty. Put the waterer on a brick, glue a plastic funnel to the top of it so they can't sit on it -- whatever it takes.
Pasty butt is usually constipation due to dehydration. Give them extra water with an eye dropper, giving a drop along side the side of their beak and letting them draw it in, a few drops and a few times a day if you can.
Since they are weak, there is a good chance they are being kept away from the fod and water by the others. Mix up a couple of tablespoons of the chick food with just a little water to make a paste, and take the two aside and offer this only to them. Throw away any they don't eat in maybe 20 minutes. You might need a second feed station, too.
The thing about adding anything to the water is, what they need most is water, and if it tastes a little funny to them, it isn't helping.
A tiny amount of molasses is usually attractive to them because it is sweet, and it is a bit of a laxative, so you could try that.
Grease up their bottoms with Vaseline, Neosporin, Crisco, veggie oil -- whatever.
Ypu might already be ding all this -- I'm just throwing out there what occurs to me.