Hi Alovell,
Thanks for coming back to us about your mobile bills, I am sorry you're finding it difficult to find what you want.
I believe in a family plan mobile account, the numbers are collated into the same bill and so there is no way of viewing your individual usage for one number, as all of them are amalgamated into the same bill. You can see a quick overview of how much these numbers have used with the "charges type summary" on page three of the PDF, this gives you an overall amount of usage on calls, text and data of all the numbers on the bill.
If you scroll further down you can the the extended list of how much usage and when for each number, but as you said this is not the most ideal way to check the usage. I understand this must be frustrating and it's something we can take away as feedback about our bill system with family plans/joined accounts.
If you are worried about going over your allowance you can create or lower your spend cap so that you cannot go over a certain amount on your bill.
I hope this clears the subject up for you.
Thanks,
Megan_L