So to bump this thread as goalie-parents are off to Paris and London next week...
They are taking my old unlocked Nokia and picking up local sim cards for voice calls and her iPad is wifi only so no issues there but for goalie-mom's iPhone (4s locked to AT&T), if she only want's to use it for wifi only (plus iMessages to me) and not incur any charges for voice and/or data, as noted above, I have instructed her to turn on airplane mode and then turn wifi back on and that's it, right?