How rare is heart cancer in children under 10?

Vanishingly rare: Except as part of tuberous sclerosis, primary tumors in children rare exceedingly rare.

Extremely rare: Cancer in the heart is very rare. Children can have tumors in the heart, but most are what is called benign, meaning they do not spread to other organs and do not cause direct harm to the heart. It is very rare to need surgery for a heart tumor. Some other cancers in the body can spread to the heart, but this is the most rare of all heart tumors.