Simple febrile seizures can recur upto 5-6 years of age. So the doctor would have advised to give during fever only upto this age. You have to take only for three days. The benefits are still controversial but there are no side effects when you take for such a short period.
Next Steps
Dose depends on the weight and not on age of the child. So if she is 15 kg approximately, you have to give full 5mg tablet 3 times a day for 3 days, even if she has fever only for one day.
Health Tips
If she has no seizures then it's good. But if there are seizures lasting for more than 5 minutes, please see the doctor