Hello Painless hard swelling on left side of chest is usually bony in origin most commonly from rib.As it is painless chances of any acute condition like costochondritis, fracture or hematoma(collection of blood due to unnoticed trauma) is very less. Even floating rib syndrome is unlikely. Some conditions particularly involving bone and presenting as lump like eosinophilic granuloma,cystic bone changes can be there,but it is uncommon in this age group. I would suggest you to consult a pediatrician for clinical examination and an imaging may be needed to see the character of bony swelling. Regards