Final answer:
Direct observation is the ideal method for documenting children's attention spans. It entails watching children in their usual environment without intervention and is better suited than self-report methods like interviews or questionnaires for this purpose.
Step-by-step explanation:
The observation method that can be used to document children's attention spans is direct observation. This method involves studying the subjects within their natural environment without interference from the researchers. An example of this method is the Strange Situation by Mary Ainsworth, a structured observation approach used to evaluate attachment styles between an infant and a caregiver, wherein the child's behavior in various scenarios is observed and documented.
Given the context of observing attention spans in children, conducting an interview or using a questionnaire may not yield the most accurate information due to the subjective nature of self-reporting or reporting by caregivers. Moreover, while experiments can also be used to study attention spans by setting up specific controlled scenarios, they might not fully capture the natural attention span as direct observation does.