Hello Darlene,
- Most of the baby bones are comprised of cartilage scaffold( soft mould)which later gets stronger by deposition of calcium and bone forming cells.
- There are growing centers at each ends of the bone called growth plates. These plates keep depositing bone cells and calcium and other minerals.
- They grow longitudinally and horizontally as well. The Bones get stronger with daily activities and stresses borne by them.
-This transformation from cartilage to mature bone is monitored by various parameters like:
-Diet
-exposure to sunlight for Vitamin D
-hormone- growth hormone, thyroid and parathyroid hormones etc
-genetic make up
-calcium and phosphorous levels in the body.
-presence of any systemic disease.

Hope that solved your doubt.