I don't see an obvious way to share just one subscription. The wording of Apple's Support article seems to indicate that there is a singe switch to "Share Eligible Subscriptions".
Add a family member to your shared subscriptions - Apple Support
"When you share subscriptions, each person sees their own preferences, recommendations, and shared content. When you share an iCloud+ plan, family members can't see each others’ photos, files, or documents."
Share iCloud+ with your Family Sharing group - Apple Support
"When you share an iCloud+ subscription, everyone in the Family Sharing group uses their own Apple Account on their device. That way your photos, documents, and other information stay private, even though you're sharing access to the service. You simply share the features and space available in iCloud+ with your family members, so you only have one plan to manage."