If your hired Disney buggy/ stroller does go missing, don't worry. Just take the receipt to the stroller rental in either park, and they will give you another one. I assume it would be the same with wheelchairs, but they are not so likely to be taken.
So - hotels....
Obviously the best location is the Disneyland Hotel built over the main park entrance, and just a short walk to the Studios park, but this is the most expensive. You would also have to use a lift as the restaurants are on the first floor, along with the bar,cafe, lounge, shop. I am not sure if they have any rooms on the ground floor.
My suggestion would be the Hotel New York if you can afford it, for a number of reasons -
This is the next closest hotel to the parks, and the next down in price. About 7 mins walk to both parks through the Village, and passing the railway station where you can buy drinks, sandwiches and snacks more cheaply to take into the parks with you. No glass bottles or alcohol allowed. You can refill drinks bottles at the drinking water fountains in both parks, which are usually near the loos. Some people take some neat squash with them, so they can dilute this for kids during the day.
The New York is the most adult hotel, not so full of families with young kids as the budget hotels, so nicer for MIL. Hot and cold buffet breakfast, and the restaurants, bar, lounge, shop, reception, lobby, pools, are all on the ground floor with no steps to worry about. Manhattan restaurant is usually less busy in the mornings than the Parkside Diner, and more comfortable.
Rooms are slightly larger than the Moderate and budget hotels, with two Queen sized or one King size bed. No kettle or teamaking, so take your own, or you can request one at reception and pay a deposit. I take a small travel one, with plastic mugs, teaspoon and hot drinks sachets which don't need milk. You casn get milk pots and sugar sachets from the fast food restaurants, or from the breakfast buffet though.
If you requested rooms on the ground floor, you would not have to use a lift at all. ( say that MIL or one of you has a fear of lifts and cannot use them! ).
I know that room 144 in Gramercy Park wing has patio doors leading out to a garden and lake. Some rooms face the rear of the hotel, where you walk to the parks, with lake view, although on the ground floor this may be obscured by trees and the artificial skating rink outside the HNY. ( open all year round ).
You may be able to get connecting rooms with one King size bed on one, and two Queens in the other.
I stayed here last summer with my eldest son, and am booked in again for a few days in October with my hubby.
The shuttle bus to the railway station for the village and parks leaves from the front of the hotel, but it is just as easy to walk, especially with kids and buggies. It is also quite nice walking through the Village in the morning when it is empty for Extra Magic Hours in the Disneyland Park from 8am.
See my detailed review with some photos from last August.
www.tripadvisor.co.uk/ShowUserReviews-g226865-d262678-r76193518-Disney_s_Hotel_New_York-Marne_la_Vallee_Seine_et_Marne_Ile_de_France.html
If the New York is too expensive, my next suggestion would be the Sequoia Lodge, which is not far from the New York, so only a few more mins walk. This hotel has a nice cosy National Park theme, with decor in wood and stone. Room are either in the main building, or in 5 separate lodge buildings. Guests in the lodges have to walk outside to the main building for breakfast and services.
The downside is that the restaurants are on lower level from reception, bar, shop, and rooms, so you have to use the stairs or lift. You also exit to walk to the parks from this floor. Shuttle buses leave from outside the front of the hotel, on the higher level.
Sequoia rooms have been recently refurbished with a subtle Bambi theme. They began with the lodges, and are now doing rooms in the main building as far as I know. Again, take your own kettle, or ask at reception.
Breakfast is only cold buffet, although last time I stayed there you could order a hot dish from a menu for about ? 6. The cold buffets in the 3 Moderate hotels are better than in the Santa Fe and Cheyenne budget ones, with more choices.
You are shown to your table, as you are in the New York and Newport Bay hotels. In the Cheyenne and SF you find your own, and they can be very crowded, and more cafeteria like.
These are photos of the new rooms in the SL.
www.dlrptoday.com/2011/02/03/bambi-adds-a-fresh-disney-touch-to-refurbished-rooms-at-sequoia-lodge/
Again, most rooms have two double beds, but some also have one King size. Rooms are slightly smaller than the HNY, and do not have a wardobe - just drawers and a hanging rail. The sink unit is also outside the bathroom, ( like in the Newport Bay ) whereas in the HNY the bath/ shower, loo, sink are all in the bathroom.
All rooms have a safe ( no charge ) which is under the sink unit.
With MIL I would request rooms in the main building, near reception and facilities, as some rooms can be a long walk away. You can pay extra for a lakeview room.
The indoor pool has a slide which is fun for kids. The outdoor pool will be open in summer months, as in the other hotels. Santa Fe and Cheyenne hotels do not have a pool. Guests cannot use other hotel pools, only their own.
I did write a review after our stay there in December 2009, but it would be better to read more recent ones.
www.tripadvisor.co.uk/ShowUserReviews-g226865-d262682-r51370552-Disney_s_Sequoia_Lodge-Marne_la_Vallee_Seine_et_Marne_Ile_de_France.html
The Newport Bay is very nice as well, but a longer walk for MIL as at the opposite end of the lake to the HNY. You would need to use the shuttle buses which go about every 10 mins.
The 5 associated hotels are further away, and do not have EMH in the mornings. The Kyriad is the cheapest one. Some have indoor pools. The Explorers and Magic Circus are popular with families with young kids. Dream Castle is a bit like the Disneyland Hotel, with Castle theming. Family rooms have one kingsize bed and bunks, so may not be suitable for you.
Shuttle buses for the village and parks go less frequently to/ from these hotels, about every 15-20 mins, and stop at each one, so can take some time.
Being able to walk to your hotel easily saves queueing for busy buses when parks close, or in the mornings, when there can be pushing and shoving from some bad mannered people!
Hope this helps as a starting point.