My daughter was like that with the carseat for awhile. I just made here sit there and let her scream because there was no way I'd give in.

I think what I'd do is do a whole bunch of short trips - 5-10 minutes at most. Maybe every day for a week. Make her wear it, ignore the screaming, and have a short ride. Then helmet off, praise for wearing it, praise for screaming less today or whatever. I'm pretty sure that she'll give up the screaming in a day or two if once she realizes that it's not having an impact.

I liked the idea of having the helmet in the house as a toy for awhile as well. You could both wear them while you cook dinner or play with other toys together, LOL... Glad I don't have toddlers in the house anymore....