Does "one-third of these children" refer to "one-third of these children who have had the first ..."

Does "one-third of these children" refer to "one-third of these children who have had the first seizure"?

Context:

How common are febrile seizures?

Approximately one in every 25 children will have at least one febrile seizure, and more than one-third of these children will have additional febrile seizures before they outgrow the tendency to have them. Febrile seizures usually occur in children between the ages of 6 months and 5 years and are particularly common in toddlers. Children rarely develop their first febrile seizure before the age of 6 months or after 3 years of age. The older a child is when the first febrile seizure occurs, the less likely that child is to have more.

More:
http://www.ninds.nih.gov/disorders/febrile_seizures/detail_febrile_seizures.htm