I will tell you what US and EU WILL do if and when Russia nuke Kyiv with a 20-50kt tactical nuclear device.
First of all, there are going to be EU or NATO peacekeeper in and around Kyiv, that's a given. That's 100% it will happen
Then high chance (between 70-90%) NATO will enforce a no fly zone to stop every air traffic in Ukraine. That's highly likely in order to contain the nuclear fall out.
Medium chance (about 50-60%) NATO will enact a peacemaking mission, declaring all Russian force within pre-2022 or 2014 border as legitimate target. And will deploy troop and air asset on the ground and pound the Russian force in Ukraine international border, every target will be destroyed. That is done so NATO can insert peacekeeper to enforce border security between Ukraine and Russia and stop the ongoing conflict. Stop the conflict from getting further out of hand.
Low chance (Between 20% to 40%) NATO will fly into Russia and destroy strategic target or nuclear capable target, also known as "Full Count" to stop further nuclear escalation.
What Russia will do is either take lost in Ukraine, or escalate to global nuclear option, as long as NATO does not have ground intrusion into Russia, this is highly unlikely to happen.
Problem is, once you use a tactical nuclear device, then NATO have to intervene. Because either NATO did not do anything, and Russia will use the same trick on other country, or even NATO member, and second, most likely the radioactive fallout will float into Poland and Poland will initiate Article 5. So NATO are going to do something, as far as I know from what I served in NATO capacity, those are the option they have.