Children can't nor have to understand our current society with their culture standards.
When you say to your kid that he has to behave the kid will see it as a parental requirement.
If you say to the kid he has to behave else he won't get anything from Santa Claus or Sinterklaas the kid sees it as something magical.
People have always known this, why else would there be fairy tales (and there morals).
You would suggest that children may not believe in these stories anymore...
I believe those children would grow up very depressed and with a great dislike of society.
You can only know this if you've ever see the sparkle into a kids eye when he opens up his presents given by some magic and compare it with the times you give him a present yourself...
Trust me, it's worth it

And for why people would dress up, well I think it's to enforce the story (children believe what they see). Off course it is also part of the tradition and as I've said: How is it racists when a "so called slave" is singing laughing and giving candy to happy children??? I think if you live in the past anything can be called racists at some point. The word has been misused a little to much.
Also, I hope everyone realizes that I (and I think a lot of other people) didn't even knew what a moor was until the entire zwarte piet discussion started :p
okay i will dress up as Hitler, parade around the streets of Amsterdam giving sweets to children and that will make my dress sense okay?
I personally think zwarte piet resembles racist stereo types of the past and given the time in history when the character came about and the pictures of him acting as sinter klass's servant, maybe just maybe the anti zwarte piet movement have a point...... but then again the kids get their sweets....