A straightforward explanation does not have to be morbid. You can create a simple comparison of a loved one's death to the death of your children's pet for example. Did their favorite dog die because of sickness? Use such scenario as a direct representation of what your loved one went through.
Explain further deeper realities that those who died can no longer come back to life no matter how hard your children may pray for it to happen. You will never know how children take in or process what you have just told them. It is important then to ask how they feel. Give them a pen and paper to draw or express what is happening inside them. Recognize any negative feeling they might share like anger, fear or sadness. Take time to talk them through the process of sorting out their feelings. Your support is very important to them as they seek for security.