The child could be suffering from BCG lymphadenitis. Please read the article on BCG vaccination from www.pediatriconcall.com or from the link given below:
http://www.pediatriconcall.com/bcg_vaccine.htm
Regarding safety of Bactrim - any antibiotic has adverse effects. Even is a patient has negative history of sulfa reaction, he/she may still develop other adverse reactions such as Skin rash: (Stevens-Johnson syndrome, exfoliative dermatitis); renal failure; neutropenia; nausea, vomiting, anorexia, photosensitivity and hepatitis. It is still recommended in children for treatment of UTI, diarrhea, toxoplasmosis and PCP pneumonia. Each antibiotic has to be prescribed under doctor's guidance.