I used to take my daughter for rides when she was 4,on a short board,planing,gybing and even jumping.She loved it when we went fast.
She sometimes goes with her brother ,exploring bays and reefs and finding somewhere new to swim.
Now she is 5,she rides on her own.
Elli is a strong swimmer,but a life jacket is must.The photo of her is in Bonaire,she doesn't wear a life jacket there as the water is no deeper than 2 feet.The biggest danger I feel when doubling a small child is the danger of dropping your rig on them,the leverage and force of the mast near the foot could crush and break bones,so I wouldn't advise novices to double young kids.But it is a fantastic way for them to experience the thrill and inspire them to become windsurfers.