I'm someone that believes not enough parents take financial (and emotional) responsibility for their children; I think we've all met deadbeat parents, I know I have (and my friends and family here say it's worse here in Argentina given the rampant poverty too) and I had to raise myself due to this, but with all do respect to Fiscal's ex, that's a fucking insane amount of money for child support, and it would keep an entire family, let alone 1 adult and two kids who live OUTSIDE of CABA/Patagonia way above the poverty line.
And while kids should never be used as pawns in these sorts of disagreements, I have a feeling they already are (i.e. Fiscal's ex telling them how evil he is, or cheap, or saying stuff like "Your father wants to go to court so he doesn't have to pay for anything!", etc.) and if she's already saying this shit you might as well actually deny her the ability to take a single centavo she isn't entitled to, because you're gonna be the bad guy either way.
Also, there's many problems I have with the Argentina "justice" system, but one of the good things here is the right of children to know their parents/have them involved in their lives beyond financial support. She can try to prevent Fiscal from seeing them, but a court can force her to adhere to a visitation regime often more forcefully than in the US.
Finally, I would follow other peoples' advice, demand receipts for everything, document everything, build a giant papertrail and digitize it. If she can't justify expenses and won't relent then use the tools you have at your disposal turn up the heat on her and her mom. She's paying 55K/month in rent for a house she owns to her mom? Ok, denounce her mother to AFIP for not registering the contract and then her for fraud if the deed to the home is fully or partially in her name. Watch how quick that will have to change to expenses for things which must be documented like electricity or repairs, not adding a second story. She's spending 250K a month on food? Your kids better be getting Don Julio flown to them for lunch, because there's no way that is accurate. That is 4 adult meals a day delivered by Rappi, every day, for a month in CABA. Where is the money going? AFIP and the UIF would probably like to know too and they take denunicas from concerned citizens.
If you don't feed her appetite will grow stronger and will eclipse the kids' especially when they really need the money when they're teens or going to university. I'm not sure if it's possible, but you could talk to your lawyer about petitioning the court for an accountant and/or trustee to ensure the money is actually being spent on the children, not random stuff for her like trips to the spa or booze.