providing a reason for your instructions doesn't hurt anything; at worst it's useless, and at best it reinforces your authority by making people perceive you as a reasonable authority figure worthy of listening to.

In addition to what others have said, I think the very concept of 'authority figure' for most people means 'one who is obeyed without question'. The meaning of 'order' does not include a possibility of questioning it. An instruction that comes with explanations simply doesn't belong in the category of 'orders'.

This isn't specific to child-adult relations. Whenever someone is in a position to give orders, asking for justification is seen as a challenge. Reasonable or rational people do, of course, ask for and give out reasons for their orders. But this doesn't reinforce authority and obedience. It creates or reinforces cooperation between two people who are more nearly equals, than a giver and a taker of orders.

The emotional/social basis for giving orders is precisely "because I say so" - orders to establish dominance and obedience - and having to explain yourself automatically subtracts from your authority.