For Smugglers Run, it works the same way it works for every other ride: approach the CM outside the queue with ALL people in your party and tell them you'd like to use rider swap. They will need to see and may measure the child to ensure they are too short to enter the line. Tell them who will be riding first and who will be riding second. Those riding second will get a rider swap added to their magic bands. Up to 3 people can get the rider swap added, including people who are riding the first time (so if you have a group of 5 with 2 adults, 2 tall enough kids, and 1 too short kid, then 1 adult and both tall enough kids could ride first, then the second adult and those 2 kids could get the rider swap and ride the second time). Those riding first ride while adult and too short kid do whatever they want outside the line. You meet up and those with the rider swap can go through the FP line and ride. Even though FP is not currently available for Smugglers Run, there still is a FP line that is utilized by those with rider swap.
For Rise of the Resistance, every member in your party needs to be in a boarding group, which means everyone needs to scan their magic band to enter the park, then be added to a boarding group through the
MDE app. Boarding groups tend to fill up quickly after park opening. Then, you do whatever you want inside or outside the park until your boarding group is called. At that point, you approach the CM to check in for your boarding group with everyone in your party and let them know you would like to use rider swap. They will either get it set up for you or direct you to another CM who can. Rider swap will be added to the magic bands of those riding second. Then those riding first ride and when they are done, those with rider swap will enter the FP line and ride. Again, FP is not available for this ride, but there is a FP line that is used for rider swap.