Oh, @Tomizgirl13 - I was RIGHT HERE a couple of years ago! Childless, never even much of a babysitter, and no clue about kids. Disney veteran and had been a kazillion times. I have now successfully navigated a few trips with my DS and her daughter (just turned 8).
From the standpoint of a similarly clueless childless adult:
1) I'm assuming (possibly incorrectly) that you - like me - tend to strategize/think about your trips in terms of rides/# of rides you can do; I'm also assuming your kid is similar to my niece, which may not be the case. I realized very quickly that pounding at top speed from ride to ride wasn't going to fly with my niece. If there was a character (walking by, on a float/cavalcade, on a building), she wanted to see it. Now when I plan with them, if there is an opportunity to orchestrate seeing a character (e.g., breakfast), I'll often book a character meal (which - ack - sacrifices rides). You will likely not get all of the rides in that you want, and you may find that you wind up having to visit/revisit/wait in very long lines for rides or attractions that you'd have blown off during a solo trip.
2) Stroller. Trust me. You may not need it, but better to have it and not need it than need it and not have it. I will never again judge a parent with a kid that seems "older" to me in a stroller. Ever, ever, ever.
3) It's pretty much all about the MK (and the pool) for my niece. Maybe a little bit about DHS now that she's getting older, but MK is still the shizzle (note, she is not as big an animal person as some kids - I know that AK is a big winner with some kids). Beware too much trekking between parks with a kid, as she may not be able to last the whole day without breaks. Honestly, I started going in a day early so I could ensure an Epcot day for myself - a lovely solo at Epcot with a drinkiepoo and/or extensive parkhopping to zigzag from ride to ride.
4) If you have the resources, stay close to the MK (e.g., Contemporary), especially since you're planning an After Hours party (I've seen different forecasts on days of week of these - Thurs or Friday - it would be great if they do both, but I digress). That ability to walk is priceless. We're planning on doing one of these parties during an upcoming brief trip with my niece, and I am dubious she will make it through more than a few hours. Heck, I did an AH at MK with my husband 2 years ago and only made it to 11:30. (To give an idea - we're unsure if my sister is going to come in for the trip, as it depends on the After Hours dates - if it's just me, I'll stay at YC and bus or Uber it back - if they come in, we'll stay at GF and walk.)
5) I always plan a break. Always. And try to take it before it becomes apparent it's needed. It's not pretty to be in the maelstrom of a worn-out kid.
6) I try to get mobile lunch orders in before 10 am. It can take a while for my sister to run through all of the menus and pick what's likely to work for the kiddo.
7) Be flexible, accept that you will likely need to slow down, take a deep breath, and enjoy. It's very different, and I'm not going to say it won't be baffling or frustrating at times, but I am so happy to have this time with my niece.
