My 7-year-old said "shum" for "thumb" for a really, really long time. My now-5-year-old called it a "fum." I don't know why I loved these mispronunciations so much, but I did. SHUM. FUM. It was weirdly sad when they stopped -- I almost wanted to encourage them to go back to their slushy babytalk versions.
I was thinking about verbal kid-isms recently when my youngest expressed his frustration with a U.S. map jigsaw puzzle. The pieces didn't fit together very well, and he got mad about halfway through when he bumped it and all the states slid apart, earthquake-style. "I don't even like Nopelahoma!" he said, storming off.
Sadly, my kids are flying past the biffed-word stage -- who approved this "growing up" business? Where can I file a complaint? -- but I have fond memories of some of my very favorite mispronunciations.