Final answer:
The child should stand 20 feet away from the Snellen chart to assess their visual acuity.
Step-by-step explanation:
The Snellen chart is used to test visual acuity, or the clarity of a person's vision. The line with letters that subtend 5 minutes of an arc from 20 feet represents the smallest letters that a person with normal acuity should be able to read at that distance. Therefore, the child should stand 20 feet away from the Snellen chart to assess their visual acuity.