Fastpass. do you get one a day or what way does it work?
you can use your park tickets in the fastpass machines at the fastpass rides (not all are fastpass) as often as you like in your park day. It depends on how busy the parks are how many fastpass rides you can do.
In Fantasyland the only fastpass ride is Peter Pan, you can also fastpass Buzz Lightyear Lazer Blast in Discoveryland, and the Flying Carpets ride in the Studios. To be honest, with the ages of your children, I'm not sure you'll use any of the other fastpass rides.
To get a fastpass ticket for the ride of your choice, go to the entrance of the ride - nearby you will see a row of fastpass machines, with a time above them. You swipe your park tickets in the fastpass machines, and you get your park ticket and a fastpass ticket back. The fastpass ticket has a 30-minute time window on it (ie 11.30am-12noon for example). You can leave the area, do what you like, then come back within your fastpass window, go to the fastpass line at the entrance to the ride, hand over your fastpass tickets, and enter the ride in a different line to non-fastpass queuers.
The only rules regarding fastpass at DLP are these - you can't use your fastpass to ride earlier than your fastpass window (or generally later at DLP, they can be pretty strict about this). And you can't hold more than one fastpass before your fastpass window opens (so in my example above, you can't get another fastpass ticket before 11.30am for any fastpass ride). In very busy times in the parks, there is another rule which applies, but I don't think you'll need to be concerned with it if you're going at a quiet time.