Hoyt Arboretum in Portland, OR. We'll be having the wedding on the Redwood Deck. 4000 SW Fairview Blvd, Portland, OR 97221 hoytarboretum.org
Whatever you feel comfortable wearing. It would also be a good idea to bring an umbrella. Heels might be difficult if anyone had planned on wearing them because when I have looked at the photos of Hoyt, not all the trails are paved.
Please, do not. Aside from just not wanting to deal with the wild card that is kids, there are other reasons it's not the best idea. The venue is outside with no shelter. It's Portland; Portland = Rain at any moment. Bathrooms may not be readily available for...er um...emergencies. (Please, don't bring pets either.)
Unfortunately, no. Everyone is responsible for their own transportation to Oregon as well as places to stay.