Tracking family while in Europe. Is there an app?

Have everyone install this app :

https://www.mspy.com

Start by getting an inventory of the phones people have. Namely, what service do they use?

(Assuming you are from the US - maybe not:)
T-Mobile and Sprint users get free data in Europe and don't need to buy SIM cards - plus free texting and 20 cents/minute phone calls.
Verizon users can sign up for a $10/day roaming plan (or maybe $40/month for very little data - big overage charges). But Verizon phones are not locked anymore, unless they are prepaid or ancient phones, so you could buy SIM cards for them in Europe.
AT&T users can get a similar $10/day roaming plan - but their phones ARE locked. Can be unlocked by AT&T unless the phone isn't paid for (still paying per month for the phone to AT&T).

The phones themselves have GPS without any connectivity in Europe, so no SIM card or roaming service needed - but without WiFi the user of the phone won't be able to communicate.

If you are in the US, consider installing Google Hangouts to be able to make free phone calls to other US numbers while on WiFi or with mobile data from a SIM. So you could call each other or even call US landlines (handy for calling your airline or credit card company back home).

Thank you for your replies.

Andrew,
I am in the US. Thank you for the compilation of Internet provider information. I’ll take it into consideration if I decide to change my provider. Google Hangouts is also a good idea.

I don’t understand. Is there a way or an app that can monitor their GPS signal without being on the Internet? Like offline maps? With mSpy it appears the person being monitored must be on the Internet. Thanks!

I wasn't suggesting anyone in your family switch providers. I was merely listing which ones already have free data (so you won't need SIM cards) and which ones at $10/day would be more desirable for getting SIM cards. Because the EU has largely done away with roaming charges, you would need to buy only one SIM per phone, if you need to buy one.

The GPS function of the phone works with or without internet connection. If you were trying to do say mapping, you'd need the internet to download the maps to make the GPS useful - or do "offline" maps. But if you want to track someone else's phone, that phone still emits GPS without being on the internet. However, YOUR phone (to track someone) would need an internet connection for that to be useful, I'd guess.

I've never used a GPS "tracker" app but there seem to be some free ones. Get two phones and try it out. Put the phone you are trying to track into airplane mode then on your phone see if you can track/find it. Search for "GPS tracker" apps. I found one simply called "GPS tracker" by googling.

I'd still find it useful for family members (you don't say how old the kids are) to be able to communicate back with the adults beyond just having a GPS tracking device (the phone) in their pockets. That's why I mentioned the other stuff.

[Hey, this is my 1,000th post! Maybe I should quit here. ;-) ]

My friend and her family use life 360. It includes GPS tracking.
https://www.life360.com/

GPS is a passive system. 24 Satellites whizzing around the earth in geosynchronous orbit each transmit a unique signal which is picked up by the aerial on each phone. By means of triangulation to several satellites the gps system of the phone (or car gps device) can work out where it is. It doesn't emit any gps signal. The only ones are from 20,000 km overhead.

What various apps can do is take the location as worked out by the device and follow that, sending the information to other devices either by cellular data or wifi data. Either way, there is no way for any phone to work out the position of any other phone other than by internet (or possibly bluetooth but only within a few feet and then you wouldn't need a tracker).

Find My iPhone works the same way, as does the find a friend part of Google Maps.

OK, now I have homework to do. :^) It’s good to know which US providers provide free data for future decision making. My phone will be on the Internet. I will try out using a GPS tracker to see how it works. I appreciate all your help.

Congrats on the 1000th post! Don’t quit now, you’re just getting warmed up.

Thank you to everybody for all the information.