I can say from experience that in the midst of a seizure it does feel like your limbs have been detached or something like that and as you can see not being a 3 year old girl even I have trouble trying to explain what its like from a lay point of view . I think it has more to do with the nerves and electrical impulses being sent to that particular part of the body as to what you would feel and even for awhile after there are still lingering feelings that are hard to put into words for people to understand .
Sometimes it just takes all your strength just to speak as your tongue and mouth are more than likely swollen from biting down on them. .
And so on . I may not have gotten all that right . but that is basically how I remember seizures.
I think your niece is brave and mature just to be able to describe all that happened to her in such a way .
And I am sorry she had to go through it

This might not be as technical as the other replies but sometimes books tell it from experience
to your niece