sheila kitznger birth crisis helpline might be a good place to start
i had read your birth story , or bits of it before, and had not realised that your other children had seen the birth.
you both , and probabvly you all, need to have some counselling to come to terms with what happened.
the birth crisis might well be a good place, as sheila kitzinger is an expert in such things and does training with the women who answer the calls. they might well have knowledge of this.
the birth trauma association might also be useful.
i think the two issues are:
you being able to debrief and process what happenend
and support DS2 in his processing of the event.
you can all support each other, and grieve together.
sorry i cannot be of more assistance, but my experience lies in counselling women who have had birth trauma and are contemplating further pregnancies, and no experience of a child traumatised by birth.
so sorry to not be more help.,but i hope the BTA or birth crisis can help