Doctor says that my nine year old has convergence insufficiency and should go for vision therapy instead of prism in her glasses. agree?

Get another opinion.: I will make a guess that the doctor who said that is an optometrist and not an ophthalmologist. Many well designed research studies have proven that vision therapy is not effective (and it's usually expensive). Please take your child to an ophthalmologist -- if there is an eye muscle imbalance, they can explain what treatments WILL be effective.