You've gotten some great suggestions!

The only thing I'd add (maybe as a last resort) is to make sure the end destination is a fun place, like the park or a stop for some ice cream. If that didn't work, I'd go so far as to have your hubby or son call from the destination on a cell phone to make sure your daughter knew what fun she was missing out on or how delicious the ice cream tasted. You know, really play it up so next time she won't want to miss out on wherever you're riding to. But really hammer the idea home that she won't be able to go unless she wears her 'big girl helmet'.

Let us know how it goes!