Are there a lot of children on cruises at this time of year (around September 20th)?
Disney cruises are never kid free, but September is definitely a lower kid season with parents not wanting to pull school age kids in the first month of the new school year. That said, since the Wish 3 nights are all Fri-Mon, parents are probably going to be more tolerant of pulling kids for a long weekend versus a cruise that requires more time off school.
Otherwise, there's another cruise that departs from Port Canaval on September 19th and lasts 5 days; the ship is the Magic (quite old, though).... It could work for us, but we'd have to make significant changes to the rest of our stay in Orlando.
We loved our stay on the Magic. It's the oldest and smaller, but we liked that smaller meant it was easier to get from one end to the other quickly. And, if you're specifically concerned about adult spaces, her adult pool deck and evening hours areas are both excellent.
Is this Wish really "unfriendly"?
So, interestingly, if you only poll first time cruisers, people really like the Wish. She's very different from the original 4 and I think a lot of
DCL regulars were unhappy with the changes because they were so used to the layouts of the older ships. Also, the fact that she only runs 3 or 4 night cruises is a big drawback for a lot of people. You'll see folks on here saying they prefer a longer sailing quite regularly and that's just not an option with the Wish.
Having sailed all 3 of the ships you're suggesting, incluing out of both ports, here's my personal opinion:
If you hadn't already set up your Orlando portion of the trip, I'd pick 5 nights on the Magic over 3 nights on any ship in the fleet. 3 nights is short and can feel like you're rushing to experience everything.
Given that this is really an Orlando trip with a bonus cruise, I'd pick the 3 night on the Wish. Getting to Canaveral is going to be much easier than Everglades and, even if she is the lowest rated Disney ship, the Wish is still really lovely with tons of fun and you get to experience Castaway.
If you do like Disney cruising, the Dream and her sister ship the Fantasy are the ones of late that do the European itineraries, so you could potentially experience that class of ship much closer to home in the future.