Oh boy, sure sounds like the Kursk operation is going swimmingly!
I mean, that's probably their only choice at this point. Back to my Ardennes comparison, its the last ditch to turn the tide. Its entirely possible they see it just the same way, there's no point holding elite troops and material in reserve because if it fails, that's it, that's all she wrote. They breakout here and force Russia to adopt a defensive posture and take the incursion seriously, or they are condemned to a war of attrition they are guaranteed to lose.
Its a huge sunk cost fallacy on human life, but as far as the calculus of war is concerned, it makes about as much sense as most of Ukraine's moves.
I haven't seen this anywhere and it's just my speculation, but I think the civilians are going to be held for prisoner swaps. I read something a few months ago that Russia had 10x the number of POWs than Ukraine to trade.
And Ukraine won't trade them 1:1. They'll be traded 5:1 or maybe higher and Russia will be in a corner where they kinda have to give in.
Outside of the war crimes issues and whether anyone with the power to condemn them will even care about said war crimes, there's two issues. One, Russia isn't really forced to do anything, if Ukraine decides to hardball and kill the civilians that's going to be a step too far for all but the most insane supporters in the west. They can go fight and win the war, liberate their kidnapped countrymen, and quite possibly use the whole thing as a recruiting push - They literally kidnapped your families, join us to get them back.
Second Issue:

Russia's attitude towards hostage taking is notoriously "You will all die if you try, the hostages are a secondary concern". Widespread kidnapping of Russian civilians is likely to be a gloves off moment when it comes to hitting civilian infrastructure in Ukraine. If you want to treat civilians as valid military objects, they'll show you how its done.