In a context of suboptimal vaccination coverage and increasing vaccine hesitancy, we aimed to study morbidity and mortality in children related to missing or incomplete meningococcal C and pneumococcal conjugate vaccines. We conducted a prospective, observational, population-based study from 2009 to 2014 in a French administrative area that included all children from age 1 month to 16 years who died before admission or were admitted to an intensive care unit for a community-onset bacterial infection. Vaccine-preventable infection was defined as an infection with an identified serotype included in the national vaccine schedule at the time of infection and occurring in a non- or incompletely vaccinated child. Death and severe sequelae were studied at hospital discharge. Frequencies of vaccine-preventable morbidity and mortality caused by meningococcus and pneumococcus were calculated. Among the 124 children with serotyped meningococcal (n = 75) or pneumococcal (n = 49) severe infections included (median age 26 months), 20 (16%) died and 12 (10%) had severe sequelae. Vaccine-preventable infections accounted for 18/124 infections (15%, 95% CI 9, 22), 5/20 deaths (25%, 95% CI 9, 49), and 3/12 severe sequelae cases (25%, 95% CI 0, 54). The vaccine schedule for meningococcal C and pneumococcal conjugate vaccinations was incomplete for 71/116 (61%) children targeted by at least one of these two vaccination programs. Mortality and morbidity rates related to vaccine-preventable meningococcal or pneumococcal infection could be reduced by one quarter with better implementation of immunisation programs. Such information could help enhance the perception of vaccine benefits and fight vaccine hesitancy.