They're both lovely resorts and we've enjoyed them both. The Yacht Club room will have two regular queen beds (instead of one regular queen bed & one pull-down queen bed in a Riviera studio), and will be more spacious (381 sq. ft. vs. 255 sq. ft). It's also in walking distance to Epcot. The Yacht Club's pool, Stormalong Bay, is amazing, with a large sand-bottom pool, pirate ship themed waterslide, and waterfall. It includes a sandy younger kids' play area and multiple hot tubs (not just one, like at most pools). It does have a deep area (the lazy river), but given that you have only one child and 4 adults, it should be easy to keep her always in sight at the pool. The Yacht Club is also the only Disney resort currently offering in-room dining. It also has more walkable dining options than Riviera, even taking into account which restaurants are currently closed. It's also either a long walk or a short drive from the Yacht Club to Fantasia Mini-Golf.
The Yacht Club wins hands down in this case. Bonus that it's less expensive.