at birth, babies don't actually have kneecaps.
In babies and children, bones have to be strong enough to support the body, but soft enough to permit continued growth. Until adulthood, when bones reach their final size and strength, they are made of a mixture of hard bone and softer cartilage. The kneecaps are no exception.
A newborn baby’s patella (kneecap) is made almost completely of cartilage. Around age 3 to 5, areas of hard bone start to form in the patella, in irregular patches. These islands of bone expand and grow together over years, gradually fusing to form a firm kneecap around age 10-12. Plenty of cartilage still remains, though, to allow the patella to continue to grow through adolescence. Sometime in late adolescence or young adulthood, the bony patches make a final expansion, replacing almost all cartilage. The result is a pair of nice, strong adult kneecaps.
Sources: http://health.howstuffworks.com/human-body/systems/musculoskeletal/babies-kneecaps.htm
http://answers.webmd.com/answers/1198626/do-babies-have-kneecaps
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