The cornerstone of fantastic parenting, stick head in the sand and then have to explain yourself when kids find new things they don't understand.
Good parenting includes explaining and teaching your children about the world BEFORE they come across it so they know how to deal with it and what things are.
Are you honestly retarded enough to think that in a playground at the age of 5 or 8 one single child hasn't overheard someone say a swear word and repeat it for all to hear.
Then on top of that, Youtube needs no censoring or checking up on, if you think its unsuitable, block it, if you don't think its unsuitable don't block it. But knowing EXACTLY what can be found on there, letting your kids use it then whining like a muppet on a forum about how bad it is. Two simple options, one of the 8 trillion ridiculously cheap or free safe for kids surfing programs, or don't let them roam free on the net.
What if one of them overheard the word sex in the playground, and decides to put sex into google, oh noes. Seriously, you think youtube is a problem?