Hi forgiveomatic. Unfortunately, the only flights with "accessible" lavatories are on twin aisle wide-body flights. Traveling between the ORD and IAH area is most likely served primarily by narrowbody aircraft. It's possible that UA has some regularly scheduled repositioning wide-body flights between the two hubs. Many airlines do. I suggest that you notify the airline's special services department in advance and they will assign you of seats as close to the lavatory as possible. Even if it is not a larger lavatory the FA's are generally very helpful about standing guard to assist in privacy issues in situations just like you present.

I don't know if your parents are able to fly first class, but I have seen people in that exact situation use the forward most lav near the cockpit and the FA will stand there and provide some privacy assistance. Now that I think of it, if you're not able to secure a wide-body flight on UA, I believe WN (that probably has great service between MDW and HOU ) is pretty good about allocating the front row for disabled passengers and that would allow them access to the forward most lav and some FA assistance.

Although your parents would likely be first on and likely last off, at least it is a relatively short flight and depending on your father's needs he may not need the lav on such a short flight.

Hopefully all goes well. Please report back on any particularly good or bad experiences!

FYI the Air Carrier Access Act (ACAA), most specifically 14 CFR 382.21 (a) (3), states that an "accessible" lavatory is only required on twin aisle aircraft delivered after 1992. I put accessible in quotes because the ACAA definition of accessible is only that one be able to maneuver the tiny onboard aisle chair into and out of the lavatory, not a large "fully equipped" restroom that would be found in areas governed by the ADA.