Children's arm bones (left) and adult arm bones (right).
In a picture of a child's arm bone (left), two bones marked in yellow are two parts of the same bone. The section marked with the blue arrow between the two sections is a rubber-like, elastic, soft-like component called the cartilage plate. This is where the bone develops as the baby grows. This cartilage disc will start to calcify (turn into bones) as the child reaches puberty and gradually merge 2 pieces of bone into a single piece. It is this that helps us to rise after puberty.
Looking at an image of an adult's upper arm bone when the cartilage plate has become a bone (right picture), we see that as the bone grows, it grows more at the ends of the cartilage disc instead of the middle.
All types of bones on the body of children, including the arms, hands, legs and feet . are developed as such. That's why the number of children's bones is greater than that of adults.
Another concrete example is the skull. The skull of the baby in the womb is divided into several different parts, this helps the baby's head to easily go out during childbirth. Then over time these bones will join together to form a stronger structure to protect the brain inside.
In short, children have more bones than adults because they need to grow up. During growing up, the bones stick together causing the number of bones to decrease. And through adulthood, the plate of synaptic cartilage has run out so we can hardly grow taller. But the strength and thickness of bones will continue to grow so we can get better.