If you are looking after the accounts for them,
here are a few ideas that may help:
Please be careful to keep the details of these giffgaff accounts separate,
you wouldn't want to top-up the wrong one!
You must log in to the right one...
An idea which may help, is to use a different device, or browser,
that way the details are kept separate.
Using a suitable name and different avatar will give you a visual cue as well.
Careful with names, especially for youngsters,
don't give personal info, gender, etc.
You could carefully initial the back of each giffgaff SIM,
so you can tell when they are ever out of their phones.
If you know how they will be using their phones,
it might be a wise time to explain goodybags to them 🙂