An article the May issue of JPS attempted to answer the question of the benefit of pre-contrast enema reduction antibiotics. The authors performed a retrospective, cohort study comparing outcome of patients form two different institutions; one that routinely gives pre-reduction antibiotics and one that does not.
The main end-points of this study were the incidence of post-reduction fever (as a marker of bacteremia) as well as adverse reactions to antibiotic administration. The authors noted no significant difference in either, suggesting (despite the usual shortcomings of a retrospective study) that antibiotics are not necessary.
Although it would take a prospective randomized study to better answer this question, it does not seem to make sense to give antibiotics before enema reduction.
Reference: Al-Tokhais et al. Antibiotics administration before enema reduction of intussusception: is it necessary? JPS (2012);47:928