How do you share a changing hospital roster with your caregiver?

My on-call roster comes out a month at a time and changes at short notice. My partner's lab days are more regular, but experiments sometimes run late. Our caregiver covers the gap between our son's school and whichever of us gets home first. At the moment I send her a message whenever something changes, which means she gets three or four messages a week, sometimes late at night. She has never complained, but she did miss one change last week and our son waited at the school gate for twenty minutes. What system do other families with shift work use with their caregivers? Shared calendar, paper on the fridge, something else?