You could try reducing the data used,
HD video uses a huge amount of data!
On the phone, you could switch to low data,
but that would be very annoying for everyone!
Your daughters may not like it,
but ask them nicely to set preferences for YouTube at a lower resolution,
once they understand it is costing you £20 a week, maybe they'll chip in 🙂
Use the cog, lower right on YouTube:
720 HD (probably half the data of 1080)
480 <--- Try this resolution (about half, or less of 720)
144 <--- This, if using for sound only (don't need video)
FaceBook, auto play of videos needs to be off
(you decide if you want to watch and eat up your data).
They all need to turn off Background App Refresh.
Which brings you to a tricky part,
as your phone is using giffgaff mobile data,
but your daughters are using it as a hot spot,
there is probably no way for their phones to tell
that the signal is mobile, as they get your WiFi.
Once their phones see WiFi
(even though it isn't really, as far as you are concerned),
their big updates will take advantage.
If it was only your phone, you could restrict updates to WiFi only,
(which you don't have at home,
so you'll have to wait for a real WiFi hot spot, when this crisis is over).
So, could each of their phones have a giffgaff SIM?