There's kidnapping and then there's fantasy kidnapping.

Since one can have you thrown in jail, I'd make sure that you are doing everything you can to protect yourself, while giving her a birthday fantasy she is seeking.

If she's into rough play, you might want to kidnap and keep her in her own home or yours.

If she's looking for some light fun, having her wear a blindfold while you whisk her out for an enchanted evening of fun may be an easy and safe answer.

I like the idea of you getting a consent form, however, that implies that she can't speak for herself if the two of you get caught. That might be a fantasy trip you are not willing to take.

Good luck and let us know how it goes!

PS

Here's one of those checklists: http://www.vampirespet.com/ActivityChecklist.html