Dear friend, welcome to HCM!
Your child had a clear post-traumatic pleural effusion- ususally a bloody one called Haemothorax.
If it was not bloody, it is still a likely late effusion.
It is likely infected and hence the fever
and vomitting. Surprisingly culture failed to show any growth- which is possible and not rare.
Starting anti TB treatment seems unreasonable here and you should take second opinion
We do not start ATT without demonstrating tuberculosis
And yours is a case of post traumatic effusion or and empyema
Try consulting a pulmonologist also and keep him on antibiotic cover. Please get a complete blood work done.
It could turn serious unless treated appropriately. TB is a rare possibility in this situation.
Take care and please keep me informed of your progress.
I hope to have answered your query satisfactorily.