Office now wants us in on Mondays, which is the one day our nanny has off

Our nanny has been with us almost two years. When she started, Monday was the obvious day off for her because I worked from home that day. From August my team is in the office Mondays and Thursdays, no exceptions. The 4 year old is at preschool till 12.30 and his sister gets home at 3.30. My partner can work from home some days, not all, and Mondays are when the partner's client calls bunch up. There's no family nearby. I could ask her to swap her day off, but it's been Monday for two years and she has her own routines around it. Paying someone else just for Mondays means a stranger one day a week. If you've had to ask a nanny to change her weekly day off, how did you put it, and did you pay extra for the inconvenience?