Since you say you are flying United, let us assume that the itin is YYZ-ORD-OGG.
There should be no customs and immigrations formalities at ORD, as YYZ is a pre-clearance airport, and CBP matters will be handled in Toronto. Based on FlightStats.com and similar sites, it looks like most of the time, United flights from YYZ arrive at the B or C gates (Terminal 1), and most of the time, the flight for OGG departs from the B or C gates as well.
As such, under good conditions, 55 minutes is not unreasonable, but conservatively speaking there is no time to dilly-dally. The United terminals at ORD are crowded, and if you have to cross between the B and C concourses, you need to traverse the tunnel. I would advise them to wait to get food, use the restroom, etc. until they have arrived near the gate of their departing flight, or unless they are very familiar with ORD and the distances involved.
If they have physical mobility problems, they can request an electric cart to take them from the arriving flight to the connection. This is done through the airline, not the airport, so they should contact the airline the flight is booked through and have the request added to their reservation.
The most likely problem as I see it is winter weather affecting operations in Chicago, Toronto, or both. A storm like the one hitting today will result in the cancellation or diversion of a significant proportion of flights, but a smaller storm could still produce delays, and a delay of even fifteen minutes could make the connection very challenging. If a storm is forecast for their travel dates, they should call the airline as soon as possible and see if plans can be altered. For major storms, airlines may also waive change fees and other charges if they want to reschedule their trip altogether.
United is the only airline that flies ORD-OGG, and on the current schedule this flight only operates once a week, so should your grandparents reach Chicago but misconnect, they will face re-routing anyway, most likely via the West Coast or possibly via Honolulu. They will not face any additional charges for the re-routing; however, as your linked article indicates, neither are they entitled to any compensation from the airline.