Get 2 fastpasses for Soarin' and double your riding pleasure! Get the fastpass, come back when you can ride and take the FP to the CM in the FP line, tell them you want a child swap, they will want to see the child, get the swap ticket, go on the ride, come back out, switch with your partner and you have only used one FP. (Wow what a run-on sentence!!) Then, if you want, you can go again right away, or wait and do something else with your little one and come back and use your other FP and child swap strategy.
We have used it that way many times and love that it doubles our FP collecting. This way we get to go on the ride a couple of times.
If the ride doesn't have FP, just tell the CM that you need a child swap and they will hand you a ticket and one of you rides and the other waits with your little one, then you switch.
One other thing we did was get a FP for everyone in our group, whether they were tall enough to ride or not. Then we usually had one or two extra FP's and could go a couple of extra times if we wanted to. It worked out great.