Her ex is most likely building wealth across his portfolio, and will be able to draw down different bits at different points. OP gives no indication that her kids are going without anything essential in the meantime.
My DH is retired now, but was a high earner and I imagine on a similar salary to OP’s ex for most of his career. I was also a reasonably high earner though went part time after having dc.
We always lived well within our means, saving and investing as much as we could. We didn’t spend money just because we had it - the kids had a sensible number of toys, mostly second hand clothes from charity shops, and we only replaced things when they broke or no longer fit.
Over the decades, with time in the stock market, our investments have grown substantially - more than quadrupling the money we actually saved. And it’s the money we were able to save 20 years ago when the kids were little that has made the most difference.
We’ve ended up in the incredibly fortunate position of being able to put both our kids through university with no student debt. Two years ago we gave our DD £200,000 to buy a house, and another £50,000 to do it up. Our DS will get the same amount when he’s ready to settle down and buy.
Neither of them begrudges the times in their childhoods when they didn't get the trending toy of a particular month, or specific items of clothing to match passing trends. They understand that our frugal lifestyle back then built the wealth that gives them the comfortable, secure lifestyle they now enjoy.