I volunteer with grieving children at Tomorrow's Rainbow (www.tomorrowsrainbow.org), a non-profit and non-denominational organization that helps to facilitate grief among children who have had a loved one die.
First of all, I commend you for coming forward to ask for this help. Most teachers are not provided with information on how to speak to children about death, so it is really great that you're asking for this information. I also suggest you find a resource near you that supports grieving children for more information.
In my volunteering, I have worked with children as young as 3 and as old as 12. When dealing with children, it is important to use the real words to describe death and dying. Do not use euphemisms such as "passed away" or "went to sleep". Simply say, "Miss Student Aide died. Does anyone know what died means?" Then, use simple words to describe death - "her body stopped working. It means that she does not eat, talk, poop, sleep, walk anymore." A great descriptor is "She is like a turtle that is not in the shell" and if you have a turtle shell, you can use it to explain death this way. You can tell the children that they will not see her anymore.
At that point, each child will ask you questions if they need more information. Some children may ask, "What happens after you die?" It is important to reply, "What do you think happens?" and let them answer. If they press to know what you think happens, you can say, "In my family we believe X, but every family believes differently. You should talk to your family about what they believe."
Children do not stay in their grief for long and at 3-5 years of age, many will "forget" and will need to be reminded that the teacher's aide is gone. This is normal. The most important thing for children to know is that it is okay to be sad and that death is a normal part of life.
Every child is different and I encourage you to continue to educate yourself on children's grief so that you can help others. Check out the resources for The Dougy Center and any children's grief support networks in your area for additional training and advice.