Hope I'm not asking too much. I appreciate any tips.

I am helping my sister and her daughters plan their cell phone needs for a 2-week trip to Germany in 3 weeks.

All 3 of them have iphone 6's. I am trying to determine whether they should get their phones unlocked (not from Apple, since they've had them less than 2 years) and buy German SIM cards, or use AT&T's International plan. It provides:

$30
120MB
Overage:
$0.25/MB 300MB

$60
300MB
Overage:
$0.20/MB 800MB

$120
800MB
Overage:
$0.15/MB

The problem is I have no idea how much data is used by various apps.

My sister needs to be able to navigate (e.g. using Apple Maps or Waze). I have no idea how many MBs of GBs one would need for that, so I don't know whether the plan would offer enough. Or if a German SIM card and xGB pre-paid calling card would be needed (which has much more overhead - getting the phone unlocked, buying the SIM card and pre-paid calling card in Germany). Any ideas?

Not to mention that I have no idea how costs compare (How much to get phone unlocked? What is the cost of a nano SIM card? How much is a pre-paid calling card?)

If her daughters can get away with just email, would the plan above provide enough data? I assume if they want instagram, skype, you tube, that they need to go the SIM card route.

My sister and her daughters would need to call (and text) each other. If my sister gets a German SIM and her daughters don't, then calling her daughters would be an international call (between my sister's German number and her daughters' US numbers) as would texting. Is it possible/practical to pay for international calls/texts on a German pre-paid card?

And, Where is the best place to get a German SIM card at FRA - Frankfurt airport?

Thanks!