Ideas? what medical condition can cause a childs penis not to grow?

Couple of issues: Short or minimal penile growth can be a feature of klinefelter syndrome, XXY syndrome or a disorder of the pituitary gland that controls the hormones that help form the genitals.

Hypopituitarism: Also bilateral loss or absence of testicles. Boys penises normally do not grow much in prepuberty. Best to consult with pediatric endocrinologist or urologist.