I would think it's no problem if you are on the same account, if not that would be tricky then. Perhaps do it via the website? You'd probably have to know their login info though.

If you are also do for an upgrade then you could use yours to get an iPhone but don't activate then when christmas comes activate it on their phone number. Then after christmas use their upgrade for a phone you want (and pay for it of course) and activate under your number. I honestly have no idea if that would work, I'm just tossing out ideas...