Quail chicks die because of several reasons. Either the temp is too cold, too hot, there are drafts, they cannot get to or find the food and or water, chicks are killing each other, animals are killing them or they have some disease.
Take a good look at your brooder. 90 degrees might be too cold for week old quail. Make sure you are brooding them in a brooder with solid sides so there is no drafts. Don't crowd them in so they have to compete for food/water. Sprinkle food all on the brooder floor so they know where it is. Crowding them in can cause cannibalism, so give them as much room as possible. Keep a lid on the brooder at all times.
If they have a disease handed down to them, there is little you can do. Keep the water and food areas DRY to keep the cocci disease from getting them. If you have a cocci outbreak, raise the quail on wire so they have less contact with the poop.
You can put liquid vitamins in the water if you think they are stressed and need a boost.
