The only thing I would add is that you need to have the too short child with you when you go up to get your switch ticket added to your
MDE. If you have more than just 2 riders and the too short child, 2 others can get extra rides. We have a 2 year old who is about to take her 3 trip to WDW.
The practical steps look like this:
1) Go up to the line (FP+ or standby depending on if you have a FP+) of the ride with your party including the too short child. A couple of lines have a specific place for rider swap, but it is marked. We found it at Slinky Dog and Flight of Passage when the line extended past the physical queue.
2) They will ask who is riding second. You can name up to 3 people and only one needs to wait. My older daughter gets double rides when she is tall enough. She rides first with her dad while I wait, and then with me while he waits.
3) They will scan the bands of the second riders to give them a Rider Swap valid in an amount of time equal to what the wait time for the group going in line should be. It is valid for an hour after it starts.
4) The waiting party takes the too short kid to wait while the first set of riders ride.
5) The people who had their band scanned go back to the FP+ line, scan their bands, and ride.