Do broken bones in the leg grow back stronger after they are break?

Not usually: A leg bone fracture will appear on x-ray to have more calcium in the healing area (for a while), but a bone's strength depends on the microscopic structure inside the bone, not just the amount of calcium. In a growing child, a well-healed bone can be stronger 6 months after the break, than before the break, because the child is now older so all his bones should be stronger than 6 months earlier.

Bone healing: Yes, a fully healed bone generally heals 3 times stronger in the area of healed fracture than non fractured bone.