S.W.
It's funny you wrote about this - I thought I was the only one with a two year old who does that! :)
Recently my daughter absolutely slathered herself and her hair with Vaseline and after repeated shampoo applications I found it just beads off. We tried applying baby oil because it's a thinner lubricant and it seemed to work after a couple of applications. Mind you her hair was a bit oily, but it wasn't as thickly coated as before, so I brushed baby powder through her hair to control the oil and after a day with that in her hair and another shampoo, it was back to normal.
Hope this helps!
S.