Spent 5 weeks here in May and June with the wife and 2 children (daughter turned 4 there and my son was 7 at the time). We also go regularly.
Rental cars are cheap, I would personally drive
Much more freedom.
A rental car is a must, cab rides in Orlando are not cheap and a lot of the hotel shuttle buses limit you on drop off and pick up times. Also outside of the theme parks there is a mountain of stuff to do and enjoy. Shopping, evening shows, restaurants etc. We rented a villa with a pool in Championsgate, and the ride to Disney was about 10 to 15 mins, the ride to Universal, Seaworld etc about 30 mins, and Busch Garden....well thats far for most people....about an hour.
This might sound super miserable, but I'm gonna be honest, I have no idea why you would take a pre-school child to Disneyland. You won't be able to get on any decent rides, they'll get fed up of queueing everywhere, they'll be more bewildered by everything than actually understanding why it's so "magical", and hell, they've barely got autobiographical memory at that point.
I'd give it a few years, but maybe that's just me.
Personally we had a completely different experience, my daughter loved all of it. Disney isnt just about the rides, its about the experience. She had an absolute blast at all of the character meet and greets. With the 5 weeks we were there, we managed to do most of them across the 4 parks. In respect of ride heights if your child is over 40 inches MOST of the rides at Disney and Universal will be open to you. My daughter just ticked this box. Some of the larger rides like the Hulk etc require 54 inches, and some of the other fast ones require 46 to 47 but as a general rule of thumb 40 inches is the magic number to enjoy most of the parks. She also loved having Breakfast at Chef Mickeys at the Contemporary Resort for her Birthday.
I seem to recall they now have a fast pass 'book in advance' thing at Disney World where you have to book the ride slot months in advance... no joke! I'd look it up if I were you.
I don't think you 'need' a car if you are just sticking to the Disney parks. Pretty good transport buses from the parks to the hotels IIRC. If you want to visit outlet malls or whatever you might want one though.
The Disney fast pass thing is a must, bur requires planning really only for a few of the rides way in advance (i.e the new Pandora stuff which we had to queue for), we had our biggest success though with just doing it on the fly in the parks on the day on our phone app. People drop and cancel fast passes all day every day and if you are quick you can pick them up on your phone quickly. In short we didnt pre book a lot of the rides way before, we did it maximum the night before but 95% on the day in the park. ...Come off a ride....boot up the app and see which rides are open in a quick time slot....walk there...job done. Also if you book way in advance it locks you in. The way it works is you can book 3 per person per day, and once they are used its one at a time. If you book all three way before, it restricts you from fast passing on the fly in the parks.One benefit of staying in a Disney property though is you can fast pass from 60 days out. Its normally only 30.
Also, avoid Hollywood Studios as it's being refurbished and as a result is currently a waste of money (I'll miss the Rockin Rollercoaster

).
Unless it has changed massively in the last month or so, the renovation isnt impacting this park much at all in my opinion. Sure Star Wars Land and Toy Story Lands are currently being constructed but it doesnt impact any of the other rides as far as their accessibility. The only one I am aware of that has been pulled down in the past couple of week is the Great Movie Ride which is being gutted for something else.....not a lot to miss there though. In respect of the Rockin Rollercoaster, I assume its the Aerosmith one? That was open when we were there, its being temporarily closed for 2 weeks in October for an update, but its 100% still there.Hollywood studios is our favourite Disney Park. Its quite small and its easy to navigate,loads of charcter meet and greets, also Fantasmic is fantastic, probably better than the Happily Ever After show at the Magic Kingdom IMO, which by the way is wayyy better than the previous Wishes one...
In respect of the weather.....we landed on May 1st and flew out to NYC on June 6th. It seemed to get hotter and more humid every day. By the time we left in June it was raining heavily with thunderstorms daily.
In respect of cost......Orlando for the 5 weeks with spending money probably cost me in the region of 15k. The worst apart about our trip was the length of the flight from India to Orlando.