We first took my DD when she was 9 months old. She has been on 4 trips since then. She is now almost 5. You shouldn't have an issue with your stroller. We have never had a stroller stolen or things taken from it. However, I would still be cautious. I don't leave anything I can't afford to lose in the stroller. I always take my wallet, cell, and camera with us when we park the stroller. I generally just leave the diaper bag with snacks, diapers, and extra clothes. As a PP noted, your stroller will often be moved by CMs attempting to keep the strollers organized. Don't freak out if it isn't where you left it.
We always take snacks and drinks in a soft-sided cooler with us so we have access to stuff for the kids. That saves a ton just in drinks alone.
Make use of the baby care centers if you are nearby. The bathrooms at Disney generally have only one changing table so there can be a line to change an infant. All the baby care centers have multiple changing tables. They are generally clean and calm.
With rider swap, everyone including the infant approaches the CM at the attraction entrance. If you have FP, you go to that line, if you don't, you go to the standby line. You ask for a child swap pass. They will hand you a paper rider switch pass. Then the baby and the non-riding adult leave the line. The person riding goes through. Later, the person who didn't' experience the attraction, can go through the FP line with the rider switch pass with up to two other people. The good news is your nephew can ride twice with each of you.