She knew at least by 6:30 PM that there would be a flight delay.
When there is a flight delay, you do not find out for awhile or sometimes even hours when you will be able to fly. The longest time for me so far, has been 3 days before we found out when we would be able to get another flight. That was because Denver airport was shut down because of snow.
There are many complications because flight crews are only allowed to work so many hours. When their shift is over, they are done even if they never took a flight.
If there is a delay, it affects flights all over the US and readjustments have to be made to flight all over. Other places may have flight issues themselves which affects planes in other places.
As it was, they only had a three hour delay, but the delay could have been for many hours. There would be no way of knowing for awhile when she would be flying.
I would think with the uncertainty of a flight delay, he would not rush out to kill the kids. I doubt if he killed them before 6:30 PM. If he did, they would have rigor.