It is widely accepted that certain people in a child's life have the right to go against parents' express wishes, to a certain point. When they have the child in their custody, they are welcome to present cookies before dinner, buy toys or clothes the child doesn't need, and generally act in a deplorable (though harmless) manner. These people include grandparents, godparents/close family friends, and aunts/uncles.
Note that nannies do not make that list.
That's fine, of course. A nanny's job is to uphold the parents' wishes, not trample all over them. As a nanny, you don't work for a family with whom you do not agree on overall child-raising practices. So you never go against The Rules.
However, there is a beautiful grey area there. While I would never flout the family rules and expectations, I will happily involve the children in projects that fall in the "parents are fine KNOWING it happens, but don't want to SEE it" category. This can more easiy be summarised as "anything messy."
The other day, the children and I were playing with shaving cream-- squishing it, painting with it, dying it, you get the idea. It was a bit of a mess, friends, but a beautiful, fun, squooshy mess of joy. After everything was cleaned up and put away, K wandered into the kitchen. Spotting the [rinsed off, innocent-looking] shaving cream cans on the counter, he looked at me with apprehension.
"What are those for?"
"Don't worry," I quickly replied. "You already missed it."
Other things that fall in the "parents can know but do not want to see" category include:
Hitting children in the face with pie pans filled with whipped cream
Enthusiastic cookie or cupcake frosting (which may or may not turn into frosting oneself) (This is not to be confused with demure cookie or cupcake frosting, which is an entirely different beast)
Equally enthusiastic baking
Jumping, splashing, or rolling in puddles
Shaving cream shenanigans
Manners-free dinners (not pictured, because all photographs show the childrens' faces-- covered in food, but still visible. Also, they're kind of gross.)