Go up to the CM at the start of the queue and they will give you a "Rider Switch Pass"
The first adult and older kids ride the ride. While Parent #2 waits with baby. You can use the stand by line, or if you have a FP you can use the FP line.
Then when the first group get off the ride, Parent #2 brings the Rider Switch pass to the FP entrance and uses it as a FP. The nice part is Parent #2 can bring the big kids along for a second ride.
Rider Switch Tips
1. Ask the CM where the ride exits. It makes it much easier to find your party.
2. Some of the rides have play areas for the younger child near the ride exit. (Splash Mountain and Mission Space come to mind)
3. Make that "wait time" special for the smaller child. Go ride something with them, get them a special treat, go see a character, or go to a kidcot station.
4. You don't have to ride the ride a second time immediately, so if you want to do the second ride at a later time, thats OK.
5. If you aren't waiting right at the ride exit, bring your cell phone and keep it on vibrate (you might not here it ring with all the ambient music) Have the riders call you when they get off the ride. I don't recommend walkie talkies. We brought them once and all we heard was other people. They weren't very helpful.