Congratulations! As a parent of twins, you might want to switch to extract brewing for a while to save yourself some time
I have a setup similar to what you're describing, but have three burners and drag it outside of the garage to use. Propane can be safely used in a well-ventilated area, but if you live someplace where it gets absurdly cold you might consider getting natural gas burners. You can get by with cracking the garage door a foot or so, but you should have a fan and CO monitors just in case. You can't be too careful.
A grinder works well for cutting open kegs from what I've heard, just make sure the keg is vented before you start cutting. If you google around you should be able to find several websites that describe how to do it. Also, if the keg type you are using has a rolled steel edge around the bottom, be sure to drill a couple of holes in it before you put it on burner. If there is any moisture in there it can turn to steam and blow a hole in the keg when you heat it up. Not safe.