A Microsoft file hosting and synchronization service.
Unfortunately, Microsoft 365 Family does not currently provide the subscription owner with a dashboard showing each family member's OneDrive storage usage.
I am afraid you would have to ask each user to check the usage and inform you, for now.
At the same time, please bookmark this page https://support.microsoft.com/en-us/accounts-billing/subscriptions/microsoft-365-subscription-sharing-faqs The documentation simply does not address whether the subscription owner can see individual or total storage consumption. However, as time goes, it can be updated to include any new information for new developments.