Most players today play with a 2-handed backhand, including on the pro tour, although most great champions have used a one-hander.

It is a decision that has to be made on a case-by-case basis. Depending on how old the player is. 11 is young enough to switch. I always say if you don't have a great 2-hander you might as well use a 1-hander; it will help your slice and your volley. Also, if you are not athletic and have slow feet or if you are going to be very tall it is recommended to switch to a 1 hander, because the 2-handed backhand requires extra footwork and extra bending down for low balls. The advantage of a 2-hander is on the return of serve and on high looping balls.