Man pages matching name sd_event_source_send_child_signal in section 3:

sd_event_source_send_child_signal.3.en	debian 11 / libelogind-dev-doc
sd_event_source_send_child_signal.3.en	debian 11 / libsystemd-dev
sd_event_source_send_child_signal.3.en	debian 12 / libelogind-dev-doc
sd_event_source_send_child_signal.3.en	debian 12 / libsystemd-dev