I think it depends on the boy.
At 6, I put my son in pleated tan pants with a belt and a blue RL button down shirt.
My husband wore the same thing except with a tie and a navy sport jacket.
My son wanted to know where his tie and jacket were? They do not sell anything like that on the ship or at port.
Next year, on the Western Carribean Cruise, I had husband and son in tuxedo's for photos with Mickey and family portrait. My son loved it. That was our Christmas photo, Greg and Mickey in tuxedoes.
If your kids don't care, long pants and a polo shirt is fine for evenings. On formal night, throw on a sport jacket!