What are you using as "formula" to feed them with? Did you use a cooking/candy thermometer to check the temperature of the formula before you fed it too them? This is extremely important, as if the formula is too hot it will burn their crops, and if it is too low, which is more common for people to mistakenly do, their crops will not empty correctly and they can develop slow-crop, crop-stasis, and more commonly they will grow yeast inside their crops as the cold formula just sits there, and this will make the babies extremely sick.
The formula that you feed them must be between 40 and 43.3 degrees C (between 104-110 degrees F), any hotter will burn their crops, any cooler will cause a host of issues with the crops emptying and becoming infected with yeast. So you must get a proper thermometer and keep a very close watch on the temperature of the formula while you're feeding it to them.
The brooder temperature should be between 31 degrees C and 34 degrees C, as they have not yet developed all of their down feathers.
If you've set-up a box of some sort instead of a proper brooder, this is fine, but the way you should do it is by hanging an ambient thermometer inside the back half of the box, and have the back half of the box be the warm half that is between 31-34 degrees C, while the front half is cooler so they have an area to come out to if they want to cool down. You do this by putting the back half of the box on top of an electric heating pad, and then covering that back half of the box with a towel or blanket, and hanging the thermometer in that back half area. The front half of the box will not be on top of the heating pad, and will not be covered. This way they have the option.
As far as the formula you are feeding them, if it's a proper commercially sold baby bird hand-feeding formula that's great, it just must be heated and maintained at the proper temperatures listed above, not optional. If you are using something else, such as baby formula, like Cerelac, then you need to add some crucial things to the formula, as human baby formulas are not adequate for baby birds.