Can I pre-download iOS update for iPad?
When iOS is updated, an .ipsw file that is downloaded from Apple's servers by iTunes. You can find direct links to them at iDownloadBlog.

At your parents' house, to update iOS, plug their iPad into a computer and launch iTunes. Then click it in the left sidebar and hold down the &uArr; Shift key while clicking on the Restore button. This brings up an open file dialog; simply look for the .ipsw file you downloaded and open it. 

iTunes will tell you that the iPad will be wiped and restored, just click OK and give it time to update. You'll be asked whether you want to start again or restore from a backup when the process finishes; choose the newest backup and the previous data on the device will be loaded back on.