OP look at this years finances, you can download them from your bank, then sort them out into different categories, to see if you can save money anywhere. Maybe DH doesn't want to be wasteful, he sees what money gets spent on, if you've refused to cut back, has he given this as an alternative?
How old is eldest DC, could they not be responsible for keeping the other two occupied till you get home?
Obviously work out how much you'd make, if you have to use childcare, deduct this from your earnings and see what the difference is.
Do you know how much was left between DH & DC? Do you know how much DH has had so far? Maybe FIL sees the spending, thinking my DW didn't work really hard to have it used like this. It's an insult to her memory. The only thing I would be worried about, is that it's in FIL name, so should be need care / God forbid pass away, how would you access the money.
Focus on your finances, see what you can reasonably cut down, see if that makes any difference at all. What did you do before DC were born?
In an excel spreadsheet, maybe split it down monthly, then you have food shopping, essential shopping, non essential shopping. Obvious gas / electric / council tax / water / TV licence / mortgage / phone-broadband come under bills. I would check that you are on the best tariffs. Mobile phone wise, do all the children have phones, can DH or yourself not get a new phone, but go onto a sim only deal. Netflix and stuff like that is a luxury. Same as eating out plus other activities like cinema, days out.
Try and budget as best as you can. A good example is Amazon Prime & Netflix, you cant watch both. Do you buy enough from Amazon to save on posting? What you buy from Amazon, is it essential or stuff you can wait for.
Other expenditure like birthdays and Christmas, see if you can shave money off that.
I will say I don't understand how Aldi & Lidl are cheaper than say Morrison's or Asda. Consider shopping around for deals. B&M are good for food / random presents / household stuff.
I think if you change your mindset you could save enough so you didn't have to work. I'm guessing a big thing is not dipping into the inheritance pot. Yes it's there, at the same time if FIL sees you with your Able & Cole box (hypothetically speaking) plus other none essentials, he will want you to not just pay for your lifestyle.
Any help at all please DM me.
Best of luck.