Right--the strollers are borrowed (deposit charged to KTTW, removed when stroller returned). There are also wagons available at Castaway for free. A stroller can be kept for a few hours (one excursion?) or the length of the cruise. As above poster noted, they are big. You might want to consider bringing your own umbrella stroller for the airports and on the ship and getting a big one for excursions.
There are a limited number available and they may run out on some cruises.