*Always* ask the CM at the front of each attraction for the procedure (different rides have a different way of doing this). For example, while you ride Space Mountain, Dad will be handed a rider-switch pass and must wait with the kids outside of the building (this is the same for RnR). Kids too small to ride will not be allowed into the cue line, they cannot go into the building. When you come off the ride and re-group, Dad will then take his pass & enter through the FP lane.
For BTMR I believe the switch process happens with the second rider & kids waiting near the exit, just outside the doorway, to enter through the exit when the 1st rider comes out (although this may have changed, so please consult with a CM).
Some of the attractions may allow the entire party to stay together throughout the line, then place the party waiting with kids elsewhere just before boarding (I think Dinosaur, for example, may do this.
At Safaris, it's rare we'll have a rider switch, but when we do...the entire party stays together in line, then are diverted to the handicap-access dock, where one parent waits with the kids until the truck returns...then they switch.
