Hi, welcome to BYC!
Sorry for your loss.
So many possibilities... where did you get them and how old were they? How long have you had them?
Warm dry bedding with enough space... with chick food and water at a cooler spot... really is about all you can do for the very young ones. In addition to checking for pasty butt regularly.
For bantams I crush my feed a little extra for the first week or so. And I REALLY like to use electrolyte/vitamin water for my first drinker full. Poultry nutri drench (feed store or Amazon) is pricey but very helpful at times. Anybody who seems sleepier or weaker than others will get a drop straight to the beak. I have other liquid bird vitamins (from pet department) that work well also, but don't contain amino acids like the PND does.
If from the feed store... all their chicks are shipped. Simple electrolyte recipe to aid recovery is... 2 cups warm water (to dissolve), 2 Tablespoons brown sugar, 1/2 teaspoon EACH salt and baking soda. Stir until dissolved and serve full strength. Add the liquid vitamins if you have them.
However, even when you do your best... not all chicks will make it. Sometimes you can't tell that until their yolk is absorbed and no longer influencing them.
Hope your new additions and your others thrive!