Final answer:
The best test to identify the source of painless hematochezia in a toddler would be a Meckel's scan.
Step-by-step explanation:
In this case, the best test to identify the source of painless hematochezia in a toddler would be a Meckel's scan. This test is specifically used to detect Meckel's diverticulum, a small pouch in the wall of the lower part of the small intestine that can lead to painless rectal bleeding. It involves injecting a radioactive tracer into the bloodstream and using a special camera to create images of the small intestine.