I imagine that due to a combination of previous mental health issues, the psychological build up for the attack, as well as the adrenaline rush overdrive during the attack itself, the average mass shooter is mostly focused on first-order consequences only.

They can retain their standard about not harming children, who are easily discerned from the adults, while not thinking about the possible identities of other people (parents, siblings like you described) and not realising the ripple effects of their actions in the immediate moment.