What is the right amount of warning?
This is purely opinion, as the argument between your sister and yourself shows.
Unfortunately, I believe you picked bad examples to ask about. Children learn about the world by taking chances (what your sister calls exploring.) If he doesn't fall, the child feels more capable and competent, which is good. If he does fall, he learns about adverse consequences, which is also good. (It is unlikely that he will injure himself seriously by falling from something the height of an armrest.) In many cases, your sister is correct. If it's your sister's couch, it's your sister's child, and her call to make. If it's your couch, your wishes to not to have the child walk on the arm rests should be respected. But respect for the wishes of others is not synonymous with obedience. It's based on empathy, something which even 3 year olds are beginning to be able to understand.
Some children are rowdy, some are reticent, many are somewhere in between. I think a parent has to take into account the nature of the individual child in nurturing them to be capable, confident, compassionate adults. Many parents feel that forcing a rowdy child to meet someone else's definition of "manners", if those "manners" are restrictive and unreasonable, is just teaching them to cower to authority and not think for themselves.*
Discipline is a complex affair involving a lot of time, thought, respect for the feelings of the child, consideration of the setting, culture, etc.
There is a great deal of controversy about the appropriate ways to discipline children, and parents are often confused about effective ways to set limits and instill self-control in their child... To be effective, discipline needs to be:
- given by an adult with an affective bond to the child
- consistent, close to the behaviour needing change
- perceived as ‘fair’ by the child
- developmentally and temperamentally appropriate
- self-enhancing, ie, ultimately leading to self-discipline1
The goal of discipline is not to keep children safe** or unobtrusive. It is to raise healthy (physically and mentally) adults.
*Adam Walsh was a tragic example of this. His father, wanting his child to be 'polite', taught him to obey adults. As a direct result (even his father admits this), the child was abducted, abused, and murdered.
Effective discipline for children
**My children were allowed to jump on their beds, climb on the shed roof and jump off of it, climb trees, even to jump off the house's porch roof into deep snow drifts. Yet I treated children's injuries as part of my living. Did they ever get stitches or break a bone? Yes, they all got stitches, and one child broke his thumb rolling down a hill. They are not perfect, but they are law-abiding, responsible, compassionate adults supporting themselves with meaningful work.