They aren't Cornish X birds.
No breed of chicken has leg problems when it is the size of a silkie. You've got something else going on with the legs. Injury, disease, or something else. I am going to assume you are feeding a decent diet. A diet would have to be insanely wonky to be causing issues in such a short time. You aren't feeding nothing but white rice, are you?
If you were told they are Cornish Game Hens, then all that means is that they are going to be itty bitty chickens. You've seen a Cornish Game Hen in the grocery store? Single serving chicken.