Well congratulations on your first batch, and welcome to the family. Raising chickens as a hobby has proven to be one of the best things I have ever done. Unfortunately, it can be challenging, and you have experienced probably the hardest part already, which is losing birds.
Ok - about the chicks. I am sure that you did ample research prior to getting set up, but I am going to walk you through a few basics on your brooder. Throughout their life, a chicken's "home" has a tremendous bearing on them - most notably, their health. In this case, we can rule out some environmental things first - starting with temp.
It is RECOMMENDED that the brooder be kept at 100 degrees for the first week of life, and decrease 5 degrees each week. However, what is far more important is the chicks behavior. If you see the chicks huddled together constantly, this means they are cold. If you see them panting, GASPING, holding their wings away from their body - they are too hot. You will want your brooder large enough to allow the chick to regulate their own temp to an extent. So in other words, they need to enough room to be able to come in and out of the heat source as they see fit. Even after brooding as many as I have, I still put a thermometer in the brooder intially to make sure something isnt crazy, then remove it after that and adjust temp based on their behaviors.
With that said - what temp is your brooder and what are you using as a heat source?