you have rightly assessed this as a seizure
activity and it would be difficult for even a much experienced pediatrician
as to ascertain the cause in this case as it may well look like a breath holding spell causing seizure or may be post trauma. but thankfully most of seizure are once in a lifetime events and we don't do anything actively except for a close watch on the child and hope this would be the case with your child too.
i can understand your problem but in case you must send your child to nursery, you should ask your doctor to prescribe some kind of emergency medicine that can be administered by you in case she has seizures and then train the nursery caretaker about the way its used.
in case she has further episodes of seizures she will have to undergo CT and EEG to acsertain the cause of seizure. for the time being be vigilante and inform your doctor of any seizure activity. thanks