It's also very hard to avoid - hard to avoid doing both simultaneously, even.
Watching my own nieces, it's struck me that different parts of their minds are maturing at different rates. It's not just that children are any fixed amount less intelligent than adults, and there isn't even a similar vector of factors for different children - children actually acquire capabilities at different points in time, even when they're genetically similar.
So niece A was better at math at twelve than B was at fifteen, vice versa for boating skills, vice versa for impulse control...
It comes down to personality differences, but they're affecting what we'd consider basic cognitive skills in ways you just don't see in adults.
Observing a child develop is a marvellous lesson in the incredibly fragmented nature of what we so blithely simplify into the label "intelligence"
Followup to: Strategic ignorance and plausible deniability
My in-law always says: "For children it is easier be forgiven then to get permission."
EDIT: This post is superseeded by my Book Review: Kazdin's The Everyday Parenting Toolkit I recommend reading only that. The remaining insight of this post is: Children expend more brain power on their parents than the parents on them.
I can say from experience: That is risky.
Children (esp. small ones) expend significantly more brain power on their parents than the parents on their children (your mileage may vary). I can assure you that they will notice these cases - at least some - and take that into account one way or the other.
If the children notice this they may assume that you either condone, accept, bear or ignore it. None of these has positive effects.
Possible alternative strategies:
I am influenced by The Adlerian School. Of relevance here is Striving for significance.
The testing of limits and the resulting interaction with the parent give the child a feeling of significance if the parent acknoledges the act of the child even if he doesn't agree with it. On the other hand ignoring the act of the child is negative feedback about significance.
EDIT: The asymmetry between parents and children with respect to the effectiveness of deniability can be generalized to any situation where one actor has significantly less overall information about the situation than another actor and thus might not be able to reliably estimate whether deniability is possible.
ADDED: tadamsmar pointed out that ignoring is scientifically known to be effective and the advice or rather personal expierence I have related in this post may be contraproductive (at least if applied in isolation).