A warm welcome to the community. Thanks for stopping by and posting.
It really depends on how you want to keep in contact with them. If it's the occasional text here and here then at 5p a message you may find that is the best option. You can check our costs and coverage before you go.
If they're all smartphones then a messaging app like WhatsApp could be useful. If you have free Wi-Fi then it'll work a treat. If you don't, then you'll need to purchase an EU data pass in order to access mobile web. The same can be said for Facebook and each have call features built in. Be aware of costs if you're not on Wi-Fi though.
Don't forget you'll need to make sure roaming is enabled so you can use the phones abroad. You can do this via your online account by following these steps:-
Online account > Plan and device > Manage services > Going abroad
Hope this information helps. Please let me know if you need anything else.