The commonest cause of death in diphtheritic child after few weeks of infection is:

Myocarditis and neuropathy are the most common and most serious complications. Deaths from diphtheria occur either from tracheobronchial obstruction by the pseudomembrane acutely or cardiac complications several weeks after the primary infection. Diphtheritic myocarditis is characterized by a rapid, thready pulse; indistinct hea sounds, ST-T wave changes, conduction abnormalities, dysrhythmias, or cardiac failure; hepatomegaly; and fluid retention. Myocardial dysfunction may occur from 2 to 40 days after the onset of pharyngitis. Myocarditis is typically associated with dysrhythmia of the conduction tract and dilated cardiomyopathy. Ref: Jauch E.C., Barbabella S.P., Fernandez F.J., Knoop K.J. (2010). Chapter 5. Ear, Nose, and Throat Conditions. In K.J. Knoop, L.B. Stack, A.B. Storrow, R.J. Thurman (Eds), The Atlas of Emergency Medicine, 3e.