D.W.
I think what you're requesting is completely honorable, and I think it's completely appropriate to make that request.
If I were doing it, I'd see if there is a public meeting room at the hospital where they were treated and have the event there so as many of the doctors, nurses, etc. would be able to attend and commemorate all the nurturing they gave your twins to help them thrive and achieve this milestone.
In the invitation, I'd simply state something like, "please do not feel obligated to bring gifts. However, if you'd like to bring something, we ask you to make a donation to __________ charity in lieu of gifts"
As far as themes - I'm blank on that one because 1 year-old birthdays are so tough, definitely more for the adults than the babies.
Good luck - I hope your fundraiser is successful.