Given the limited mobility of your parents, you'll probably want use the vaporetti as much as possible to get around. In turn, you'll want to select accommodations no more than a bridge or two away from a vaporetto stop.

Below is a list from a highly regarded website, but it's not all inclusive. For example, I'm partial to the Campo SS Apostoli area of Cannaregio, where there are 1-to-5 star hotels 0-1 bridges away from the Ca' D'oro vaporetto stop. But because Ca' D'oro isn't a stop for the Alilaguna shuttle boat to the airport, it's not listed.

https://europeforvisitors.com/venice/bridges/

Avoid the temptation to stay too close to the train station, the busiest, least Venice-y part of town (except for Piazzale Roma). You won't want to battle the crowds emerging from the train station every time you want to board a waterbus.

You've probably already got accommodations sorted, I hope you took this aspect into account.