6 year old has white round callous skin patch on bottom of big toe & center feels tough. ulcer? feet tingle frequently. diabetic?

At that age....: It is likely a plantar wart. See a podiatrist to be sure.

Depends on the cause: Many things can cause calluses. Some reasons that one may develop a callus include: viral infections (warts), friction, pressure, repedetive trauma, and a limitation of joint motion. Some genetic conditions may also cause people to develop calluses on the hands and the feet. Treatment depends on the cause of the problem. Have it evaluated by a professional and get the appropriate treatment.