I do apologise for being so vague in the first place. I was trying to strike the balance between confidential information and helpful information. I also wanted a clear conscience about the information I'd given if DH objected to me asking Mumsnet.
This thread has helped me, despite all the crying. We are short each month of £××××, mainly due to pre-existing contracts that we can't get out of. I did think that I could "borrow" £xxxx from the £xx,xxx of DH's pot or from the £xxx,xxx of the entire pot. Then, pay it back when the kids are older and I can get a proper full-time job. I no longer think this. Several posters have highlighted why this is a very bad idea, not least that it might not be possible.
As for the job, I know and admire many working mums. Their lives are often very hard. Part-time working mums generally still do the vast majority of the housework, childcare arrangements, birthday arrangements, everything. I have seen kids who enjoy going to childcare and I have seen children in tears when they realise they're going to childcare.
My 3 primary school aged children have never done this. I have been there in person 99.9% of the time. It will be a very big change for them and none of them seen keen. We have no family nearby to back us up.
I will look carefully at jobs but I will admit it's daunting. I had planned to do more hours when the older two were in secondary school but my hand is being forced. Termtime jobs seem the only option. 3 days a week is great for holidays but will need childcare during termtime. 5 short days is great for termtime but how would we manage during holidays? We'll be able to afford to buy holidays but have no time to take them.
Hence the question: should I forget about the inheritance, upend our lives and get more hours? Or insist DH sort out FIL and use the money we've been generously left to give everyone an easy, happy life until it's a better time to get a job? £xxxx out of £xxx,xxx makes it all go away.