*ETA:
Lots of Europeans, speak English. They are multi-lingual. So know that.
When I was there w/my Husband, I often just went cruising around by myself and was fine.
Also, make sure that your daughters, don't get caught up in the savvy gorgeous men there. MEN. Or young men. If your daughters are street-smart, then they won't get carried away or hung up on oogling the guys/men there. They are at an age... in which, they notice men/good looking men/boys. And they can be a lot more savvy or street smart, then them. And hopefully, your daughters are not the "partying" types who will go with a bunch of men to a party if they are invited, because they are Tourists.
I mean, here in Hawaii, I often see young women here on vacation... and they often fraternize with the handsome local men or go night clubbing... and think they are "friends." Which they are not. Local men... KNOW tourist women. They stand out.
It is just common sense.
Tell your daughters, do NOT separate.
And make sure, your daughters KNOW how to use the Euros. And pay for things. Its not hard.
AND make sure, that they AND you... have the phone numbers to the Consulates or Embassies.
You said they are making their own plans.... and you are not entirely sure, where they are going/where they are staying/etc. So you must, make sure. Have a copy of their itinerary, their lodging info/phone numbers/contact, their flight info. etc. It is not a matter of you being a micro-manager parent. It is just common sense. Just in case. If it were your Husband going, you'd know all that info. So same for your daughters.
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I didn't read all the responses here yet.
Anyway, per cell phones... you HAVE to make sure, it can be used in Europe. Or not.
Ask the Provider.
And, does your daughter, KNOW how to use it in Europe? The "area codes" are different etc., when calls are made there to outside the country.
Like any traveling, make sure she has some cash on her.
Euros and American money.
American money for using while they are still on lay overs etc. at the American airports and/or for tipping etc.
Euros for when they hit European soil.
KNOW that, in Europe, there are lots of pick pockets.
Wear a waist pack or inside-your-waist type travelers pouches or whatnot.
Look online for travel pouches and for their I.D's etc. and passports.
Are they going with a tour group?
Or just your daughters by themselves?
You said they are making their own plans...... so, what is their itinerary etc.? You need a copy of that. Too.
If they have an iPhone or Smart phone, they should also be able to do "online/internet" things on it. If there is WiFi around where they are. And thus, they will be able to contact you either via phone or Skype or Face Time etc.
Again, make sure that their cell phones, CAN be used in Europe.
Some can. Some can't. And depending on the plan you have and the type of phone.
Before they leave, make sure you/them makes copies of ALL their passports/credit cards/medical insurance cards etc.
Just in case.
Make copies of everything, both the front and backsides of it.
And you, keep the copies. At home.
I have traveled to Europe and my Husband is from there.