In a child with a pre-existing cardio valvular disease, antibiotic prophylaxis should be administered before a dental extraction so as to prevent:
Premedication with a systemic background

Patients with Subacute Bacterial Endocarditis - antibiotic prophylaxis is needed.
Patients with abscess - antibiotic therapy.

In a child with a pre-existing cardio valvular disease, antibiotic prophylaxis should be  administered before a dental extraction so as to prevent bacterial endocarditis.
Reference: Pediatric Dentistry SECOND EDITION, Shobha Tandon; page no 163
Bacterial endocarditis
Congestive cardiac failure
Pneumonia
Rheumatic carditis