Contractions

When I teach contractions to 2nd or 3rd graders I usually start with the two words. For example, I write can not on the board. Then I write the words next to each other: cannot. Finally I cross out some letters and add the apostrophe, explaining that the apostrophe takes the place of the letters that are removed: can't.

What is ... what's
It is ... it's
Would not ... wouldn't
Could not ... couldn't
Cannot ... can't
Do not ... don't
Did not ... didn't

So what's the deal with won't? Shouldn't will not become willn't?