Is it possible to allocate fixed amount of storage to members in an iCloud Family?
You cannot allocate storage from the shared pool. However:
If each member gets their own 50GB but you have a shared payment method specified (do you pay for applications they purchase? if so then you have this turned on) then all the iCloud Storage bills will go straight to you.
My recommendation is to try sharing 200GB first - if you've already tried this (your writing seems to indicate you haven't) or if it immediately fails then you could split it up - but you may be overestimating how much any individual person's storage will grow based on what's available.