Even with a pipe on the exhaust, and a blower on the pipe for negative air pressure to suck it out, you're going to get exhaust gas buildup in an enclosed garage; unless you have some pretty big holes in it. Back when I was working on the shop floors, in the winter time, that wasn't a big problem as we'd be moving trucks and equipment in and out every hour or so, and the roll up doors let a big exchange of fresh air in. But there were still too many times (once or twice each winter) we'd have guys with bright red faces and splitting headaches (signs of CO poisoning) from doing engine tests inside and the doors hadn't been opened in a while, even though the exhaust system was running full out.
Only thing you can do then is transport them to the base Emergency Room for oxygen. And they'd be off the floor for a day or two until fully recovered. Then we'd have the OIC hammering us about safety for the next 2 weeks.
If the garage is connected to the house, don't run it inside.
If the garage is separated, open the big doors first before going inside if it's running inside. Locking the doors is sufficient for safety/liability
If it's under a car port, or open shed, or even totally outdoors, it should be in some kind of fenced in area with a locking gate so no kids can just wander in. (Just like a lot of towns have swimming pool regulations to keep kids from wandering in and drowning.)
If you want to secure it from theft while it's running unattended, best thing I've seen is a logging chain wrapped around the axle and padlocked to a ring set in concrete in the floor/pad. And I recall that in the old days with antique tractors, they'd pull a rear wheel off and run a belt over the axle as an erzatz PTO to drive equipment; which makes driving the tractor off kind of hard.