There is a service Disney World offers which is a VIP (very important person) tour of the parks. The charge is by the hour and not by the person. It's expensive; it's like $300 an hour. You pick the amount of time you want their service. The tour involves a Disney personal guide who will take you through the park. An added value to this tour/service is you get the best seats for live shows and parades and fireworks. It does not say anything about faster access to the rides. I'm sure this guide would get the fast passes you need and maybe help you somewhat on ride lines, but there's no guarantee on this part. This requires a minimum of 72 hours advance notice to book it. If you consider the cost of the trip this may be worth it; so, you don't lose out.
There are other personal tour guide companies available in Orlando that purport of getting you through the lines faster. I don't believe they do anything more than getting fastpasses and/or a GAC. You would have to do a search for these outside companies and call to see what they can do for your group and the cost for it. They would be a pair of eyes for your group.
I do believe getting a GAC may help you in some cases, but not all. How capable is your one child who is not autistic? Can that person get fastpasses? The Disney cast members are at the fast pass machines and they will help with getting the passes if you want them to help. Try it on your own the first day and see how it goes.
Finally, there are caregivers you can hire for about $20 an hour. They may not have any skills when it comes to Disney World, but they certainly can be a set of eyes for you. You would have to pay their park entrance fees in addition to their pay per hour. They get a driving/petro allowance, too. Also, it's expected to pay for the caregiver's meals while they are with you. You might want to ask for someone who has been to Disney World and maybe, you will get one who knows the ropes. This would probably be the cheapest way if you can't do it for yourself. The caregiver is willing to meet you at your resort.
If you were satisfied with
Disneyland Paris I would think you should be okay with Disney World. I don't think it will be easy, but hopefully you will come away with some good memories. Disney World is trying a new system called fastpasses plus and you are able to pre-do your fastpasses before you even enter the park. You get a wristband with RFID with all your info on it. Maybe, you will be lucky enough by the time you get there that this new fastpass system will be more in action. It's being tested in the parks, right now as we speak, but it's not across the board yet. I believe it's being done randomly, right now. You may want to do a search about this new system to learn about it.
I do have a concern for you because there are some rides at Disney World that have walking conveyors that are moving while you get on the ride. Disney can slow these down, but they can't always stop them for you to have access standing still. There are not too many of these rides, but be careful. Another drawback is the Animal Kingdom Park has rough terrain for its walkways ( not smooth ) through the park. In other words, you may have some tripping hazards, here. Also, Magic Kingdom has some curbs you have to watch out for especially around the castle.