That's not a word you want in your vocabulary when caring and watching your spawn fly around on all fours, getting into and doing things they shouldn't. There is the pain that they feel when their little feet slip out from under them and the only thing left to cushion their fall is a head that is so disproportionate to the rest of their body that gravity acts quickly making sure that it is the first thing to meet the ground. Then, there is the pain the parent feels seeing the crocodile tears stream down the face knowing the new found agony they just felt. Then, and let's all be honest with ourselves here, there is  the pain of having people notice black on blue knots on your kids head who give that look of, "wow, dad, nice parenting". 
I have a strong conviction about this though. Even though it's painful for both me and my 1/10 scale version of me, I let it happen.
"You LET your child experience pain?!" (maybe this guy actually is a bad dad)

Please read me right one this one. I don't cause my child pain and I don't allow her to get in situations that would detrimental to her health. But I do let her hurt herself.
"My God! Why?!"
There is an old adage that says, "Anything that does not kill you, only makes you stronger", and I think there is a lot to be said in that. Us humans are pretty dang stubborn and it seems that we have a tough time learning things when the going is "going good". But I find it absolutely amazing how many "life lessons" and "street smart" ideals I've learned from the times in my life where going has just been a plain pain in the butt.
For instance: We all have that "coffee table of death". The one that sits right at the end of the couch and is the last thing that separates the living room from the dining room or the hallway. How many times have you gotten close to running into it? How many times have you told yourself that you should take that corner a little wider? Sure. We can tell ourselves that something "will probably hurt", but something about just telling yourself isn't very effective. 
But then comes the frightful, barefoot day when you actually do take that corner too close. And out of nowhere the coffee table inches out its little leg JUST far enough to clip your little, vulnerable pinkie toe, trying to separate that poor little bugger clean off of your foot. I bet you take that corner a little wider every time now, don't you?
For another instance: Love. We've all felt it, we've all been hurt by it. I can't even begin to tell you how many times I, with no reservation, threw my heart out to a girl. And how, every time, that girl took my heart and launched it with a force of a bat hitting a baseball during fly-ball practice. Each time I was hurt, each time I felt pain, I learned more what true love was, and more how to better handle and approach giving my heart away. I know I love my wife as much as I do BECAUSE of the pain I felt, not in spite of it.
"I see where you're going with this."
I don't like Niamonster feeling pain, but I also know that when she is learning to walk, the times she falls down help her learn how to better balance. The times her head is plastered to the wall as the only friction slowing down her decent to the ground, she is learning physics and gravity. When she crawls under the table and sits up too fast and bonks her head, she learns spacial reference. When she flings toys around and one nails her in her fat melon, she learns force and Newton's laws.
She won't get up and exclaim, "Daddy! I know Newton's third law now!"... and in fact every time she bonks her head on something, even though she may be kissing schools such as Harvard and Princeton goodbye, I know she is "getting it". She's learning a lot fast then if I would pull her away and protect her from every little bump and bruise. 
I also know that protecting her as she grows up will get tougher and tougher. Right now, she bonks her little peanut head, but someday she'll be a teenager who has a bruised heart from some boy. I want my little girl to grow into being a woman who can handle her own, who knows she can heal, who knows she can get up the next morning and continue on with life. Who knows that pain is just another step closer to being a better person. 
Pain will happen. Growth will happen. And I know that there is nothing I can do to stop it.