Hmmm... not sure I can give specific experiences, but here's my tip... we have a DD4 and DS7. DD4 is still too small to go on most of the big rides, but DS7 can ride everything. We like to combo fastpass with the child swap. We'll use two tickets to get a fastpass for TOT (for example) and then the other two tickets to get fastpass for the RnRC. When we come back for our fastpass time for TOT, we ask for a childswap ticket. DH and DS will go through the fastpass line with their fastpasses and then when they get back, I'll take DS and go through the fastpass line again - using the childswap ticket... or I'll save the childswap ticket and use it at another time or on another day - those childswap tickets are GREAT because they are like a fastpass that doesn't expire and can be used for multiple riders (up to four, I think).
The other benefit of how we use the childswap passes is that we can have multiple fast passes going on at once - since the fastpass system only lets you have one at a time.