Plumbing and Piping - bathroom sink drain stopper corroded away

My child's bathroom sink drain stopper has corroded and collapsed and is no longer functioning. I had to pry it out of the sink and when i took out the stopper, there was a thick "guck" for lack of a better word, all around the inside of the drain. What is this "guck" and how do i get rid of it? Why did the drain stopper corrode and collapse?
Oh - I should mention that it is not an old house - about 6 years old

Hi kafay,
Welcome to the DIY Forums.
The stopper may have corroded and collapsed due to many various reasons ranging from the construction/composition to a caustic substance left sitting on it. The "gunk" in the drain most likely is soap,toothpaste,,shampoo or anything that got poured down the drain. This builds up and leaves a residue on the sides of the drain pipe. To clean it off you can use a bottle brush and a little soap. You can buy replacement stoppers assemblies for most all stopper kinds.

Good Luck and reply back if you have further questions.

thanks - i'll certainly take your advice and use a brush to scrub it out.
On another note - we are thinking about replacing our kitchen sink and faucet. I notice you are a plumber. Is it a DIY thing or a hire out. If we hire out, what can we expect to be charged by a plumber?

You can replace a sink. When you get ready to do it, we will be here for any info you need. Good luck.