I saw your question dated May 22, Tuesday just now ( June 29 Friday).
Though not appropriate to you now, as this is in public forum, it may of some use to others.
Sudden onset of fever is likely to be viral fever
. Before concluding that kindly go through the following.
1. How is the childs activity? After giving paracetamol calpol
the activity should increase for few hours
2. After giving calpol fever should come down a bit for atleast few hours.
If these two things do not happen then a visit to the doctor is called for.
What are the additional symptoms?
1. Running nose?
3. Ear Pain?
5. Urine colour brown? Pain while passing urine?
6. Motion consistency change? Any blood in the stool?
7. Any skin rash? Particularly red or purple rashes
8. Any swelling anywhere.
9. Any swallowing problem, pain in the throat
10. Any glands in the neck.
Generally viral fevers last between 4 to 8 days. In otherwords after 96 hours fever should settledown. So persistent fever, as such, is acceptable.
Hope there is no source infection like elders at home having running nose or cough or fever