What is of concern to a paediatric endocrinologist is if this early breast development is just a precocious thelarche or a pointer to a serious underling condition e.g if a child has a tumour in the liver producing estrogen, breast development will occur.
Now, the final line is to see a paediatrician who will examine and investigate for pathologic causes of early development..... and once this is absent, there is no need to worry. Breast development is under the influence of Estrogen, which is a hormone. Usually, the biologic clock gets activated at a particular age, usually 8yrs in girls and 9yrs in boys. So the first obvious sign of puberty in a girl is breast development while the first sign of puberty in boys is increase in testicular volume.