Erin, I believe your story and yes the Wiggles are awfully odd, so I thought I'd check it out on google, the source of all truth. First I got the MSN Encarta dictionary, as follows:

slip·per·y dip (plural slip·per·y dips)
noun
Australia children’s slide: a long children’s slide in a playground or funfair

Then I found this REALLY weird australian website
dolly story
with this story about some dolls, and someone with obviously way too much time on their hands.

There's also some powerpoint about slipperydips which I can't open cause we dont have powerpoint installed on this computer...
slippery-dip powerpoint

Then, finally, there is some story about seeing ghost-children use a slippery dip, cached off of google.
haunted slippery-dip

Great stuff, that google.