I guess there's no need to freeze them, just keep them dormant, so I guess this question doesn't apply. My thought was to put them into clean milk and into the fridge, would this do or is it risky?
Yes, you can refrigerate them for up to three weeks. Refrigeration slows down the fermentation without killing the culture.
Use a jar with a tight lid. This will prevent cross-contamination with other flavors in the fridge. If you have other cultures there as well, such as yogurt, that's an additional reason for the tight lid.
Use a large enough quantity of milk. You will need at least two cups of milk per tablespoon of grains. You want to put in enough that the milk sugars aren't entirely processed before you can switch out the milk. If you put in too little milk, the bacteria will covert all the sugars, then run out of food and die.
Leave adequate headspace, around two inches. The tight lid will prevent the carbon dioxide from escaping. So if you fill the jar too high, a lot of pressure will build up and in theory, the jar could explode. Or at least, the kefir will spill out when you open the jar, as soda from a shaken can does.
You might need a couple of milk changes after your return to get your grains thriving again. But I've refrigerated grains for two weeks or so and they always jump right back. Also, the kefir that results from this long refrigerated fermenting is eminently drinkable. Due to the slow action and the tight lid, it's not as tart and somewhat more carbonated than the countertop stuff.