This page will tell you what devices are connected to your wifi:
I'm sure others on this site will give you some more tips.
Most smart phones have a setting on them in which you can see how much data is being used via WiFi. It will differ with each type of operating system the phone has, so it would be best to search how to see your WiFi data for the particular operating systems your phones have. There are also apps available for smart phones which can tell you what's using data on the phone, though, again, they differ by operating system.
The other thing you should do is ensure that only the phones are accessing your WiFi. You can see what is connected, and what has been connected, but is no longer, since the last time the modem was plugged in, here.
The following two pages may also help.
Edit: If your phones are Android based, this may help with regard to seeing how much data is being used.
Maratsade had mentioned Glasswire, which is a program that is very often recommended for those with Windows based PCs who are having data usage issues. It appears that they now have a version for Android based phones, as well. If you have Android based phones, it might be exactly what you're looking for. I can vouch for its value when it comes to Windows PCs, and if it's anything like that, it should work very well for your phone. It will not only tell you how much data your phone is using, but also just what's using it on that phone, as in built in apps and 3rd party apps, as well as system processes.
You can read about it here, on the Glasswire site.
Good morning Tracy123,
I'm glad you found the community, thank you for posting. I hope the suggestions maratsade and GabeU offered helped you out! It's been a while since we last heard from you, so we will close this thread. If you still have concerns, please start a new thread and include a detailed explanation so we can better assist you.