Go on them with her! Honestly it's a blast they do allow single riders but that usually means you get paired up with another single rider. Not a terrible deal but rides like that are more fun with someone you know.
There are some height restrictions but most are within the 40 inch range.
Length of stay and where you stay are entirely dependent on your budget and when you want to be there. There's busy times and quiet. So research what's going on at the parks during the time you want to be there. For example if you were wanting to go in Nov./dec. Magic Kingdom closes at 6pm some nights and reopens for a hard ticket event called Mickey's Magic Christmas. Those you have to buy into seperately.
If you don't mind renting a car there's a lot of cool stuff to do outside of the resorts too. I would recommend the Orlando eye experience! Universal and Sea World were also a lot of fun as well.
Be prepared for a lot of walking, the parks are huge.
Be prepared for longer lines, it's not uncommon to wait for an hour or more for some attractions.
If you can afford to, buy fast pass. You'll still have to wait but not nearly as long as if you didn't have fast pass.
Bring water and some snacks in a backpack. Park food is expensive and it adds up pretty quick. Also sunscreen and hats.
Stay for the happily ever after fireworks at 10pm, but get yourself to the front of the castle and find a place to sit about an hour before. You'll never ever see a display like happily ever after ever again.
Animal Kingdom is awesome. The nighttime safari is a must, we had a rhino block the road in front of us right next to pride rock and there were three or four lions hanging out at that moment, it was so cool.
I'd also recommend buying a memory maker photo pass card. It's a card you carry around and give to the onsite photographers when you meet characters or for when you get off rides that had a photo mid ride. Your pictures are downloaded to your card and you have up to 30 days to retrieve them from it.
Download my Disney experience app for your phone, it's a really useful app at Disney, it tells you the real time waits for rides and character experiences, where washrooms are, where food is, where character experiences are located. it can be linked to your memory maker card as well so you can see your photos as they upload.
Lastly, take me with you, I can't stress that enough!
There's a whole bunch of other stuff as well but it's your trip, you guys make it about what you want to see.