Would you accept this interim "acting head" work role

[BTshun]

Gauging opinions..

For context: my manager James ("Head of Contracts")'s manager Charlotte ("European Head of Legal") is going on maternity leave for 6 months. Contracts is a separate function that falls within Legal)
James will be moving into her role (Acting European Head of Legal) for 6 months.

The nutshell of my dilemma is that James has asked me to move into his role for 6 months - Acting Head of Contracts - and I don't want to.

I've been doing my job for a long time and do it well. I work a 4 day week. I'm the most senior member of the Contracts team apart from James. I get on well with James and there is good mutual respect.
Even if James left the company, I wouldn't be applying for his role.
It's a thankless role - he does the same role as me but with the added responsibility of managing several other people plus a lot of global work outside the immediate role.
I have youngish DC at home (prep school age). My husband works in a highly stressful environment with longer hours than me so all of school pick ups and drop offs, appointments, school concerts etc fall to me. I'm also trying to manage a house renovation project. I've lots going on in my life. I don't want this added responsibility or stress. Especially as I don't consider his role to be my "next step career wise". Career-wise I like my job and I'll keep doing it. I'm not looking to step up any further, certainly for the next few years until the DC are more independent ie I no longer have to take them to school. My current role fits in with my lifestyle. I have good work/life balance, lots of flexibility and earn very well. The Acting Head role wouldn't come with a salary increase but money isn't my driver these days because I value my W/L balance and flexibility more than any increase in salary would give me.

The issue is more than James ss expecting me to say yes. As is Charlotte. I'm probably the only one with enough experience to take it on.
If I don't do it, James will probably keep the dual roles and resent me because his workload has now more or less doubled. He might also start making my life more difficult because I've made his more so by not accepting the role.

I don't know how I can turn down the role and keep my good relationship with him (and this is important to me), broadly intact.

Any thoughts on how I should phrase this to not come out of this looking like a poor team player.

I’d just express my thanks for their faith in me and that I am very flattered but the timing isn’t right for my personal circumstances on this occasion will decline the offer

Thank you. You might be right. I try to keep work life and private life separate though. Not sure why really.

Is there anyone else in the team more junior to you, for whom this role would be a step up/career progression, where you could offer to support them if they need help but be clear that you're not actually taking the role on.

You might need to support them a bit more than you'd like at the start but I'm sure they'd get up to speed quickly. In the long run, it would be good for you too - if there is someone else in the team with more experience, there would be someone to cover your work if you needed to take time off, and it would help someone else's career development but with a safety net around them if they are out of their depth initially.

Also means you're showing willing to James.