Apple could easily decide to charge $25 per family instead of per account for enabling iTunes Match, and leave it up to you how you first get the content onto each phone.
I believe you could get the content on your phone either by switching to one iTunes (original/shared) account on your phone, downloading the iTunes match music, and then switching to the other (personal) iTunes account (with match enabled) and then removing selected music from your phone (if you want to reclaim the space and have it in the cloud only).
Or you could connect to iTunes desktop and transfer the music from there.
Both of these would have the advantage of allowing each family member to maintain their own music library (not sharing all content).
I haven't tried either of these options yet, because I don't want to pay $25 times 6 ($150!) for iTunes Match for the family.
I the meantime, I'm using separate iCloud accounts for each family member, and a shared iTunes account (used for iTunes only), thus a family of 5 results in having 6 apple IDs.
I'd like to add that iTunes Match was the one feature that finally encouraged me to switch the whole family to the Apple ecosystem (5 iPhones (mom, dad, 3 kids), 2 iPads (mom + dad), 1 Macbook Pro (dad)). $25/year was a steal to get all our music library on the cloud on all devices. It's disappointing to see this feature come into conflict with Family Sharing.