'they had no need to phone as there was no change of plan on their part.'
They normally left the house to go to work. This means they needed someone to be there to look after the children. On this day, due to extreme weather conditions that made travel difficult they decided to stay at home. This means they did not need someone to come and look after the children. This is a change of plan.
They wanted her to come in as usual, but as they didn't need her to come in, and her coming in was difficult (due to the aforementioned extreme weather conditions) the reasonable and courteous thing to do would have been to call her and give her the choice.
'if pink wanted to change the plan the onus is on her to arrange it.'
She didn't want to change the plan, she was happy to struggle in to allow them to go to work, as normal. She didn't want to struggle in to find that she was not needed.
'no point them phoning to say, we still want you to come in as normal as that would be expected that she would still come in unless she had called to say otherwise.'
I would expect them to ring and say 'we're not going into work, we would still like you to come in, but if you feel that getting in would be too dangerous/difficult we don't need you today'.