Ural with a sidecar?
Bryan, I join others for commending you for doing everything in your power make your little girl happy.
While all the options proposed are solid, I feel that your daughter will not enjoy rides two-up, because her view will be obstructed by your back and she'll need to lean quite far to see what's ahead.
Slingshot is a good option, but they're expensive and I haven't seen too many positive reviews.
If I were you I'd consider a motorcycle with a sidecar. Ural makes good ones, and while expensive brand new, you can pick up a used one for a reasonable price.
Here are the benefits:
1. You'd be able to fit the sidecar with belt and other accommodations your girl needs.
2. she'll have an unobstructed view of what's ahead and around.
3. you'll be able to see her all the time
4. you can take Ural off-road - they were made for that.
5. you can fit the sidecar with a windshield, or even a complete cover with windows, like on picture below (I know it's not Ural, just one of the options).
I hope that helps
