Give her a chance to grow. Decide later whether she needs a surgical fix.
DO NOT BREED THIS DOG. Even if she seems to "outgrow" the disability. DO NOT EVER BREED THIS DOG.
As Menageriemamma says, if you bought from an ethical breeder, you should expect a refund of your purchase price. But an ethical breeder would have this spelled out in a contract, and is highly unlikely to have a sold you a pup where this problem is already detectable. Was the pup vet checked prior to sale?
Also, an ethical breeder will issue a refund without demanding the pup back -- but may require both the return of her registration papers and a spay certificate, now that there are so many "paper mills" that will "register" a dog no questions asked. So you may have to wait until your pup is old enough to be spayed, and this is entirely reasonable.
If she does require surgery for the knees, it may be prudent to spay at that time. I'm a big believer in minimizing anesthesias, especially for toy dogs and sighthounds. (One thing I love about my vet -- for example, yesterday he stitched up two nasty lacerations on my eight-year-old SAR partner's face with no chemical restraint -- just a touch of lidocaine, Momma holding things steady, a deft hand with a suture, and good training and trust. So it was no big deal -- with sedation it would have made her sick and miserable for a couple days.)
Genetic defects can crop up even when a breeder takes all reasonable precautions, but they are much more likely when they do not.
A very common problem in toy poodles (even more common in Poms, as Chirpy found out). It is painful and debilitating.