IME, @25 degrees F, you wont have to worry at all about the kegs freezing. There is lots of mass in each until they are nearly empty, and the alcohol will lower the freeze point low enough where you ought to be able to serve no problems. Move it outside at the last available moment to buy you some time. 55 might be a higher than desirable serving temperature, but it will cool as it flows through the serving line rather quickly.
If you are serving out of picnic taps, this is where things usually freeze up. I have a 5' picnic tap line to serve outdoor events and it will freeze up rather quickly, if no one pours for about 5-15 minutes dependent on ambient temp. There is so little a cross section of fluid in the tube, and the distance it needs to travel through the cold air keep serving this way rather problematic. Here's what you can do:
Get one of those neoprene sleeves that are available for hydration bladder systems and cover the hose. This will work pretty well in temps close to freezing. It will buy you time in lower temps. I've also used pipe insulation from the hardware store in a pinch but it is not as flexible and is much bulkier.
Last resort is to keep a bucket of tepid water close by and keep the hose wound up in it. This is a pain in the butt because you don't want it too warm, so you need to babysit it often, and people who are not paying attention end up tossing the whole show in the increasingly dirty water.
You may have luck with just a towel or blanket wrapped around the serving line, but honestly the best way to keep it from freezing is to SERVE. if beer is MOVING through it, it wont freeze up and everyone will have fresh cold beer.
