Two of the seriously injured were rushed to a Begusarai private hospital.
Khagaria DM Alok Ranjan Ghosh said the bombs were kept in a plastic bag in garbage collected by the ragpickers.
Sources said a few children started playing with the bombs presuming as balls when they exploded. The sound rocked the area and injured around a dozen persons in the locality.
Khagaria SP Amitesh Kumar said the incident was being investigated from all angles.