So can you do the child swap with fast passes?
I'm not sure the context of your question.
Any sort of child swap is basically so that the other members of a group can enjoy an attraction with a minimum of fuss when one or more members cannot or do not want to ride.
For attractions that have a Fastpass line, and also have a height restriction, Disney offers a Rider Switch pass, which I've described above and on my web page. It operates similar to a Fastpass, but is good for three guests (instead of just one). You can only get one (usually) if you have a child who is too small to ride however.
The Rider Switch pass allows one or more guests to wait OUTSIDE the queue area for the attraction while the first group of guests use the standby (usually - the Fastpass line can be used if you already have Fastpasses - although there MAY be some restrictions - see my web page) line. You can go shop, go to another attraction, anything to keep the little one from being bored waiting.
ANYTIME later that day (there is no return time, it is good for the entire day) the person who stayed behind (it doesn't NEED to be, but that is the intent) and up to two others can then use the Fastpass line so there is minimal additional wait. Its not as quick as if you could do a swap right at the attraction, but you don't need to wait in a long line, deal with a complicated swap at the loading area, all with a cranky kid upset they can't ride or trying to manage a stroller in a narrow queue. Instead the young child can still enjoy something else, and perhaps get you to spend money in the stores while you wait.
And besides, Disney will not allow an underheight child in the line anyways.
If all the guests who want to ride have Fastpasses, you COULD use those instead - but you have to get them in advance and it prevents you from getting FPs for another attraction for a while. But if its a case of a child who is tall enough but just doesn't WISH to ride, they can be a convenient option.