From the FAQ: https://www.nintendo.com/switch/online-service/faq/
"Can I change from an Individual Membership to a Family Membership?" - "You can purchase a Family Membership and set it for automatic renewal even if you have an active Individual Membership. This will turn off automatic renewal for your Individual Membership. However, your subscription terms won't be combined; your Individual Membership will remain active until the end of its term."
So if you buy a 12-months solo membership and two months later you buy a 12-months family membership, you seem to waste 10 months of the solo membership.
And if you plan to play with two persons together on the same Switch online (f.e. you and a local guest want to play vs. 6 other online people a round of Mario Kart 8 Deluxe), you'll need two individual subs or the family sub.
On PS4 and Xbox One only one subscription is needed if you and your guest want to play (on the same console) against others online, your sub expands to the other local players.
Thanks for the info!
I still don't plan on getting the subscription but if my daughter asks, I will get it for her. At the same time, I might want it for myself if I ever decide to try Smash online. I've owned Brawl, Melee, and 4 but I've never played a single online match.