Responses
I think Hikari is referring to the other caregiver children giving Chisa and Dai a fail and letting them pass by. And Chisa seemed to be trying for a baby herself, but had miscarriages so that would explain the hospital visits and graves.
I think Hikari is talking to the "dead children". The couple are very good parent and it's a shame that they didn't get to experience being raise by them hence the sentence "they let them pass those two".
Can someone explain me chapter 5? What does hikari's last dialogue , " you really let those 2 pass by you huh"? Mean and are those 3 children theirs that passed away? And its some sort of fertility issue that's making chisa frequent hospitals ?