Hello,
If the only symptom they have is
fever, then you are doing all you can do with giving
Tylenol for fever and keeping them well hydrated.
However, I am not sure what you mean by lethargic, but if they are not acting normally or are difficult to arouse, you should bring them to see a doctor as soon as possible to make sure they don't have an infection in the blood.
Also, if they have been having the fever for more than 3 days, I would suggest getting in to see a doctor.
Otherwise, if they are eating, drinking, and acting normally, and have only had the fever a couple days, you can continue the Tylenol and fluids and watch for any signs like I mentioned.
Hope I have answered your query. Let me know if I can assist you further.
Take care
Regards,
Dr Aaron Branch, General & Family Physician