Don't worry about it. A little cold spell won't hurt your ukes. If you've been living here for awhile you'll remember the cold spell we had about 2 years back around the same time of year where it dropped below 30 on the beach here at night. The cold weather lasted for a few days but none of my instruments had an issue and I didn't have a heater when I lived in that place! If you're worried, the best thing you can do is just keep them in their cases. Sudden changes in temp aren't great for instruments, but gradual changes are not a big deal. Just keeping them indoors should be enough of an insulator. A bigger worry is prolonged exposure to too much or too little humidity. Here in Miami too much humidity is the issue I mainly deal with.