Actually, to correct the previous post, they no longer allow children under the height limit to get into the queue. Here's how it works:
1. Entire party approaches the CM at the entrance of the attraction. Tell him/her you have a child too small to ride and you would like to "baby swap" or "rider swap."
2. CM will give you one "rider swap pass." All riders who are going on the first round then either enter the stand-by line or give their Fast-Passes to the CM and enter the FP line. Adult staying with the child(ren) then moves off to the side or goes shopping or finds a place to rest and wait.
3. After the first group exits the attraction, the adult who was left behind then takes the rider swap pass and up to 2 more riders (pass is good for 3) back to the attraction's entrance. Give RSP to the CM and enter the FP line and enjoy the attraction.
4. Meet up with the rest of the group at the exit.
Notes: (A) The RSP does not have a time limit on it, so you don't have to use it right away, you could go back later that day. (B) RSP are only available at attractions that have height limits. (C) You can only get a RSP if you have children too small to go on the attraction. They are not for instances where a child is big enough but too scared to ride.
Hope this helps!