Floyd Mayweather's Grandson KJ Steals Show at Beach Birthday Party

It was Floyd Mayweather's big day, but all eyes were on his adorable little grandson, KJ ... who cut up the sand and even sparred with Grandpa at TBE's beach birthday party!

The All-White Beach Party went down Wednesday night at Joia Beach in Miami ... the first party of 8 planned for Mayweather's 48th birthday (which is officially on Monday).

Celebrating with the 50-0 boxing legend were Jacquees and Rotimi, who performed. Lil Meech was also there, as was James McNair, Jona Rechnitz, and a bunch of other family and friends.

Floyd's grandson, 4-year-old KJ, son of Yaya and NBA Youngboy, was having a blast ... dancing on the beach, and sparring with his boxing legend grandfather while partygoers watched.

By all accounts, the party was awesome ... but there won't be too much time to rest. Mayweather has a skating party, bowling party, an ATV tour, and much more planned.

It's a welcome few days of partying for Floyd, who has been busy this year ... closing several massive real estate deals.

Floyd is also in the process of making a run at the NY Giants ... where he's planning to make a $700 million dollar offer to buy a 10% stake in the team.

Happy birthday!!