Can a dentist be certain that a 5 yr old needs a root canal on a rear molar without xrays?

Not really: If a baby tooth abscesses you often see a gum boil (fistula), but. Imwould still want an x-ray to visualize what is happening inside the tooth and in the bone. Must be treated asap to prevent damage to underlying permanent tooth.

Maybe: There may be enough signs, but i would always want an x-ray to make sure what I am dealing with.

No: He can make a clinical judgement by what he see. How badly the tooth is decayed. But an x-ray is really the standard of car for making a positive diagnosis.