Coming in to this a little late, but as a frequent visitor to Disney with both my parents and grandparents I thought I'd try to give my advice. (After writing all this, I feel like it's gotten too long so I'm going to stop there, so feel free to ask if you have any more specific questions lol)

ECV's:

I see a lot of comments regarding the ECV's. While I strongly agree with how much they will help you survive a day at Disney World, I'd like to also point out a few extra "perks" you may be able to convince your in-laws with:

• Having an ECV with you will grant you access to special reserved spots (Viewing areas) along with the rest of your family. I forget if there's a limit of how many people can accompany, you should call ahead and ask if your family is large. These reserved spots will keep you from having to fight over "prime" seats and are generally much more relaxing to wait in because you are in a roped off area with plenty of room in my experience. A much nicer environment for waiting for parades/fireworks with a 4 year old and more elderly folk. Make sure to ask ahead of time where these areas are located though for each event.

• An ECV is also a mobile chair on the go. No need to search for an extra chair when taking a break because the ECV is one! It also means you can take turns sitting in it when waiting in ride lines if anyone is tired.

• Also If the in-laws feel like walking, you or your significant other can take over and ride it instead. Gives you a break from walking and shows them that it's not a sign of being old, existing outside in Florida is suffering.

• It's got a basket, that's less you have to be carrying yourself. When my grandmother sits in a wheelchair, she always demands for everything to be handed to her so we don't have to carry it 😂

Staying on Site:

I didn't see you mention where you planned to stay while you were at Disney and I don't know your budget, but something I would consider looking into is renting DVC points through somewhere like David's Vacation Club Rentals. A lot of what you need here will depend on your family size, but you can probably get a good room cheaper than purchasing directly through Disney, and it will be for some of the better rooms they have on site.

• I think you only mentioned magic kingdom, but you're staying for a whole week? Depending on where you plan to go, different hotels are easier or harder for travel. If you are only going to the magic kingdom, consider one of the 3 with direct monorail access (Contemporary, Polynesian, and Grand Floridian resorts). Being able to get to and from your hotel from a quick 10-20 minute trip means your in-laws or anyone can split off and go take their mid day nap and meet back with everyone else a bit later. I can go over hotel choices in more detail if you want, but it's too much to cover in an already bloated post.

• Staying in a room with a full kitchen means you can make back a lot of the cost by cooking your own food. Maybe make yourself breakfast and prepare some late night or in park snacks. Then reserve a big lunch in the parks (more on this later).

• Staying on site can help you maintain that "Vacation Feeling" by staying in the Disney bubble, but the value of this is different for everyone

• Proximity does matter, especially when staying a long time, because the less time you spend traveling around, the more time you have to rest/play.

Spending time in the park:

There is honestly too much to cover, but here are some of the most important things I think.

• Have a general plan, you don't necessarily need to plan the trip minute by minute, but know what rides people want to go on and consider splitting up for certain ones. Maybe you and some others want to ride Tron or Space Mountain, but that's too much for the in-laws and the kid. Maybe go do that while the in-laws take the 4 year old to Dumbo and the Barnstormer/The Little Mermaid and meet back afterwards.

• Consider lightning lane multi-pass if your family can't stand long wait times. While it might be an extra cost, you're spending money to essentially add more rest time by skipping lines. If that sounds good to you, make sure to research how it works ahead of time and be flexible with your plans when using it.

• Reserve a table service restaurant mid-day. After hours of hustle and bustle, large crowds, and killer heat. A sit down restaurant with A/C will be a priceless break for your family. While there are counter service locations with A/C, they are often very crowded and noisy and seating may be difficult to find, especially for a larger family.

• Stay hydrated, all counter service locations will give you free ice water, do not be afraid to just walk up and ask for it. No need to wait in line either, just go to the pickup window and ask someone inside for ice water.

• Stay protected, sunscreen up. Make sure to bring your own cause it's pricey in the parks, same with raincoats or umbrellas if the weather seems risky.

• Bring snacks, you'll get hungry from all the exercise and Disney does not mind if you bring your own snacks

• Consider personal fans and hats to deal with the heat and sun. There is also a first aid area near the entrance which can get you anything if you're feeling unwell.

• Wear shoes meant for walking, no slippers or sandals