Perhaps GBC called NBC on the landline around 6.30, and he came over in BWANA. Then GBC went out driving and called NBC on the mobile at 6.57 (why), then the police at 7.15.
Unless the girls were booked into before school care, and why would they as GBC would have been able to take them to school, then I think OW got them out of the house at GBC's request and may have taken them to say Maccas for a play to use up time until she could take them to school. The youngest girl was in prep and she would have needed to be taken into her classroom and get settled for the day. So then OW drove them to school, took the youngest one into her room then came back. If she changed cars as I haven't got my head around that yet, then she did that either before or after the school drop off.