NEVER turn the big valve partially closed on a (one pipe) steam radiator. It should be totally off (turned off only when the system is cold) or totally on. To leave it partially open will prevent condensed water from flowing back through the pipe to the boiler. The water sitting in the radiator will clog the unit, destroy the steam valve, possibly spit water out of the steam valve, and eventually rust out the radiator.
The air valve at the other end of the radiator is used to regulate how quickly and for how long the radiator remains hot. It does this based upon the size of the air vent hole relative to the other radiators in the house. Big holes should be for upstairs and further away from the boiler and small holes are for downstairs and nearer the boiler. Some steam air valves have variable hole sizes on them.
Here is what I would suggest: First of all your cat is much smarter than you think. It will know that the nice place to sit or walk in the summer is not as good in the heating system.
Steam radiators on a system that is working well do get very hot (steam: over the boiling point of water >212 F.,) much hotter than a hot water system, (around 160 to 180 F.) Where this is a major concern, (usually with small children who are not as aware as your cat) radiator covers are installed. These are boxes often made of metal (though some are wood) that have a grill on the front of them. These will not get quite as hot as the radiator itself.
If you are looking for a subtle heat you should be thinking of radiant floor heating. There the temperature of the pipes is around 80 deg F. but your entire floor is the radiator not the hunk of cast iron in the corner.