As Kecanoe said, a healthy adult, a healthy parent would distract the child from this activity. And, generally speaking, this sort of thing may happen naturally when the child is much younger, 1-2 may be even 3 years old but not older than that. By the age of 8-9, in a healthy family environment, the child is well aware of the concept of private parts and the body integrity in general. If an 8-9 year old is innocently unaware of this and touches the parent's nipples out of ignorance, that means that someone in the family introduced this behavior to the child and normalized it, and that is sexual abuse. In a healthy family, the idea of doing this would simply not cross the mind of an 8-9 year old. When the child of this age does what you did in your memory, someone taught them this behavior.

The shame is not on the child who did this. The shame and the blame is on the perpetrator who tacitly or explicitly encouraged the behavior.