E.,
Sorry this is so long...I don't have seizures and neither do any of my kids, but my dad was a neurologist, so I knew what seizures were from a young age. There was a little comic book called "Seizure Man" that I used to read all the time when I was a little girl that explained seizures to young elementary-aged students. It was really informative and put them in a reasonable, not scary light. It also told the kids what to do and not do during a seizure.
I would also recommend a little role play. Just like how we teach kids to call 9-1-1 with a (uplugged) real telephone and role play, I would show and tell her what it will look like and how long they might last (if you know) and what to do if she sees Mommy on the floor...i.e. how to check your breath, how to roll you on your side (if she can) and how to call Daddy (or another grown-up who knows) if she is worried for you so they can talk her through this. You might also get a phone with the big speed dial #s for 911, fire and Daddy because she may be flustered a bit when it happens until she gets more used to it and might forget the #s.
I would also consider taking her to a neurology appointment with you so she can ask the Dr. as many questions about it as she has. He may have some literature or ideas on how to expain it too if you call him (or see if there is a pediatric neurologist near you that you can call to get ideas as well). I'm sure they both would have ideas as they deal with kids quite often...both in families with seizures and as a patients.
It also might help her to have you make a big poster of pictures for the wall that she can go to or take with her to you that reminds her in pictures of what to do when you are having a seizure. It also might help her to go to class...if she is in preschool or once she reaches Kindergarten... and teach the kids what a sieizure is (if you wouldn't find that too embarrassing). Anyway, hope something here helps. I can remember learning about seizures from about your daughter's age and as long as she is curious and asking questions, she is old enough to learn the answers.
Here are some resources I found online while googling for you...
This list has lots of resources that are available to you that you can contact the guy and he will send them to you...
http://www.efmn.org/index.asp?Type=B_BASIC&SEC=%7BFF0...
Another list, but this has it broken up about 1/2 way down into age range-approppriate books and such...
http://www.vanderbiltchildrens.com/interior.php?mid=996&a...
The seizure man comics (don't google it...some idiot decided to make some very off-color paridy of it...not cool) but here are the names and the author of it so maybe you can contact them.
-Seizure Man: First Aid for Seizures
-Seizure Man: In the Classroom
Both are done by Bowman Grey School of Medicine (now the Wake Forest University School of Medicine on the Bowman Grey Campus) so maybe contact the school and ask for the neurology department and see if they are still in print if your dr. does not know.
Good luck and let me know if you need any other ideas or if the link aren't working. Also, let me know what happens with your daughter...if any of my ideas work. :-)
S., 26, mom of 15-month-old and #2 on the way