First, all 4 of us went to the cast member working the line. (You can also approach the cast member working the fastpass line, if fastpass is available.) We told him that dad and son were wanting to ride, but we also wanted to use the rider switch. He sent the big boys through the standby line and gave me the following pass:
While the boys were riding Space Mountain, Cman and I walked around Tomorrowland, looked in the gift shop and took photos. Actually, that's what I did while he slept! Once the boys were off the ride, dad stayed with Cman while he slept and JAG and I went back to the ride. This time, we went through the fastpass line and I gave the attendant my rider switch ticket. He took my ticket and we walked right down the fastpass line and got on the ride. It was great for JAG- he was able to ride Space Mountain twice within about 25 minutes (since the standby line was so short at this time)!!
Anyone can use the rider switch program if you have a child or a dependent adult who can't ride the ride due to height or physical restrictions. You can also use the switch if 1 child is too scared, not just too short. You have to have 2 adults to use the rider switch program so no child is ever alone. One parent to ride while the other waits.
All members of you party must be present to request a rider switch pass- at least 2 adults and 1 child. If one member is missing, you will not be allowed to receive the pass.
You can also "stack" a rider switch with a fastpass. If you arrive to the ride with your fastpass ticket at the specified time, you can request a rider switch pass. In that case, the first parent goes through the fastpass line instead of standby, rides and then makes the switch. The second parent will also use the fastpass line since they have the rider switch pass.